Differential Equations in Pumas

Author

David Widmann

1 Introduction

Pumas automatically chooses a differential equation solver that is suitable for the simulation or estimation of the dynamical system of the NLME (Nonlinear Mixed Effects) model at hand. This default solver is the preferred choice and optimized for most users and use cases. Nevertheless, in some cases the performance-accuracy trade-off can be improved by adjusting the tolerances, or possibly even the algorithm, of the differential equation solver.

In this tutorial, the Warfarin PK/PD model is used to demonstrate how to configure the differential equation solver.

2 Learning Goals

  • Observe the utility of the @vars block of a Pumas model with respect to storing dynamic variables associated with differential equations
  • Understand the main differences between common differential equation solvers for nonlinear dynamical systems
  • Learn how to adjust the algorithm and the tolerances of the differential equation solver

3 Warfarin PK/PD Model

We return to the Warfarin PK/PD model. Its dynamical system consists of three states, \(\operatorname{Depot}\), \(\operatorname{Central}\), and \(\operatorname{Turnover}\), whose dynamics are governed by the ordinary differential equations:

\[ \begin{aligned} \operatorname{Depot}'(t) &= - \operatorname{Ka} \operatorname{Depot}(t),\\ \operatorname{Central}'(t) &= \operatorname{Ka} \operatorname{Depot}(t) - \frac{\operatorname{CL}}{\operatorname{Vc}} \operatorname{Central}(t),\\ \operatorname{Turnover}'(t) &= \operatorname{rin} (1 + \operatorname{emax} \frac{\operatorname{Central}(t) / \operatorname{Vc}}{\operatorname{c50} + \operatorname{Central}(t)/\operatorname{Vc}}) - \operatorname{kout} \operatorname{Turnover}(t) \end{aligned} \]

with PK parameters \(\operatorname{Ka}\) (absorption rate), \(\operatorname{CL}\) (clearance), and \(\operatorname{Vc}\) (volume of distribution) and PD parameters \(\operatorname{rin}\), \(\operatorname{emax}\), \(\operatorname{c50}\), and \(\operatorname{kout}\).

The dynamical system can be written more concisely by introducing auxiliary variables for repeated expressions:

\[ \begin{aligned} \operatorname{Depot}'(t) &= -\operatorname{ratein}(t),\\ \operatorname{Central}'(t) &= \operatorname{ratein}(t) - \operatorname{CL} \operatorname{cp}(t),\\ \operatorname{Turnover}'(t) &= \operatorname{rin} \operatorname{pd}(t) - \operatorname{kout} \operatorname{Turnover}(t) \end{aligned} \]

with influx rate \(\operatorname{ratein}(t) := \operatorname{Ka} \operatorname{Depot}(t)\), concentration \(\operatorname{cp}(t) := \operatorname{Central}(t) / \operatorname{Vc}\), and \(\operatorname{pd}(t) := 1 + \operatorname{emax} \frac{\operatorname{cp}(t)}{\operatorname{c50} + \operatorname{cp}(t)}\).

4 Auxiliary Variables in @vars

In Pumas, dynamical systems are defined in the @dynamics block inside of the @model definition. For instance, the dynamical system of the Warfarin PK/PD model can be implemented as follows:

warfarin_pkpd_model = @model begin
    ...

    @dynamics begin
        Depot' = -Ka * Depot
        Central' = Ka * Depot - CL / Vc * Central
        Turnover' =
            rin * (1 + emax * (Central / Vc) / (cp50 + Central / Vc)) - kout * Turnover
    end

    ...
end

The same concise rewriting can be applied in a Pumas @model by defining auxiliary variables (“aliases”) in the @vars block:

warfarin_pkpd_model = @model begin
    ...

    @vars begin
        cp := Central / Vc
        ratein := Ka * Depot
        pd := 1 + emax * cp / (c50 + cp)
    end

    @dynamics begin
        Depot' = -ratein
        Central' = ratein - CL * cp
        Turnover' = rin * pd - kout * Turnover
    end

    ...
end
Tip

The walrus operator (:=) ensures that the aliases do not show up in the simulation output of the model. However, if you would like to access an alias in the simulation output, you should define the alias with =. For instance, if you want to obtain concentration cp as part of the simulation output, you can change the @vars block to

@vars begin
    cp = Central / Vc
    ratein := Ka * Depot
    pd := 1 + emax * cp / (c50 + cp)
end

5 Differential Equation Solvers

The differential equation in the Warfarin model is non-linear, as detected by Pumas (“Dynamical system type: Nonlinear ODE”):

using Pumas

warfarin_pkpd_model = @model begin
    @param begin
        # PK parameters
        """
        Clearance (L/h/70kg)
        """
        pop_CL  RealDomain(lower = 0.0, init = 0.134)
        """
        Central Volume L/70kg
        """
        pop_V  RealDomain(lower = 0.0, init = 8.11)
        """
        Absorption time (h)
        """
        pop_tabs  RealDomain(lower = 0.0, init = 0.523)
        """
        Lag time (h)
        """
        pop_lag  RealDomain(lower = 0.0, init = 0.1)
        # PD parameters
        """
        Baseline
        """
        pop_e0  RealDomain(lower = 0.0, init = 100.0)
        """
        Emax
        """
        pop_emax  RealDomain(init = -1.0)
        """
        EC50
        """
        pop_c50  RealDomain(lower = 0.0, init = 1.0)
        """
        Turnover
        """
        pop_tover  RealDomain(lower = 0.0, init = 14.0)
        # Inter-individual variability
        """
          - ΩCL
          - ΩVc
          - ΩTabs
        """
        pk_Ω  PDiagDomain([0.01, 0.01, 0.01])
        """
          - Ωe0
          - Ωemax
          - Ωec50
          - Ωturn
        """
        pd_Ω  PDiagDomain([0.01, 0.01, 0.01, 0.01])
        # Residual variability
        """
        Proportional residual error for drug concentration
        """
        σ_prop  RealDomain(lower = 0.0, init = 0.00752)
        """
        Additive residual error for drug concentration (mg/L)
        """
        σ_add  RealDomain(lower = 0.0, init = 0.0661)
        """
        Additive error for PCA
        """
        σ_fx  RealDomain(lower = 0.0, init = 0.01)
    end

    @random begin
        # mean = 0, covariance = pk_Ω
        pk_η ~ MvNormal(pk_Ω)
        # mean = 0, covariance = pd_Ω
        pd_η ~ MvNormal(pd_Ω)
    end

    @covariates FSZV FSZCL

    @pre begin
        # PK
        CL = FSZCL * pop_CL * exp(pk_η[1])
        Vc = FSZV * pop_V * exp(pk_η[2])
        tabs = pop_tabs * exp(pk_η[3])
        Ka = log(2) / tabs
        # PD
        e0 = pop_e0 * exp(pd_η[1])
        emax = pop_emax * exp(pd_η[2])
        c50 = pop_c50 * exp(pd_η[3])
        tover = pop_tover * exp(pd_η[4])
        kout = log(2) / tover
        rin = e0 * kout
        time = t
    end

    @dosecontrol begin
        lags = (Depot = pop_lag,)
    end

    @init begin
        Turnover = e0
    end

    # aliases for use in @dynamics and @derived
    @vars begin
        cp := Central / Vc
        ratein := Ka * Depot
        pd := 1 + emax * cp / (c50 + cp)
    end

    @dynamics begin
        Depot' = -ratein
        Central' = ratein - CL * cp
        Turnover' = rin * pd - kout * Turnover
    end

    @derived begin
        """
        Warfarin Concentration (mg/L)
        """
        conc ~ @. Normal(cp, sqrt((σ_prop * cp)^2 + σ_add^2))
        """
        PCA
        """
        pca ~ @. Normal(Turnover, σ_fx)
    end
end
PumasModel
  Parameters: pop_CL, pop_V, pop_tabs, pop_lag, pop_e0, pop_emax, pop_c50, pop_tover, pk_Ω, pd_Ω, σ_prop, σ_add, σ_fx
  Random effects: pk_η, pd_η
  Covariates: FSZV, FSZCL
  Dynamical system variables: Depot, Central, Turnover
  Dynamical system type: Nonlinear ODE
  Derived: conc, pca
  Observed: conc, pca

Pumas approximates the solution of the differential equation with a numerical differential equation solver. Generally, one distinguishes between solvers for stiff and non-stiff differential equations.

5.1 Stiff vs. Non-Stiff Systems

A key distinction among numerical solvers is whether they are designed for stiff or non-stiff differential equations:

  • Non-Stiff Differential Equations: These systems exhibit relatively moderate changes in their variables. Standard non-stiff solvers can efficiently approximate solutions of these systems.

  • Stiff Differential Equations: These systems contain rapidly changing components alongside more slowly varying dynamics. Non-stiff solvers typically perform poorly on stiff systems, as they may require exceedingly small step sizes to maintain numerical stability. Specialized stiff solvers are therefore employed to handle the sharp gradients and large timescale differences without compromising accuracy.

6 Pumas’s Automatic Solver Selection

By default, Pumas adopts a hybrid approach with automatic stiffness detection to switch between stiff and non-stiff solvers as needed.

6.1 Model Simulation

  • Default Solvers: Rosenbrock23 (stiff) and Tsit5 (non-stiff)
  • Tolerances: Relative tolerance \(1 \times 10^{-3}\) and absolute tolerance \(1 \times 10^{-6}\)
  • Rationale: These higher (less stringent) tolerances permit faster simulations while maintaining sufficient accuracy for exploratory and predictive modeling.

6.2 Model Fitting

  • Default Solvers: Rodas5P (stiff) and Vern7 (non-stiff)
  • Tolerances: Relative tolerance \(10^{-8}\) and absolute tolerance \(10^{-12}\)
  • Rationale: These lower (more stringent) tolerances ensure high precision during parameter estimation, which is critical for matching the model’s predictions to observed data.

The default solvers and tolerances are recommended for most users in most instances. If desired, however, it is possible to adjust these settings with the diffeq_options keyword argument.

Important

Computation time decreases as tolerances are increased. However, higher tolerances come at the cost of a less strict error control, and hence generally a less accurate solution.

6.3 Adjusting the Tolerances

The absolute and relative tolerance of the solver can be specified with abstol and reltol.

7 Absolute and Relative Tolerances

When employing a numerical solver, it is necessary to specify how accurately the solution should be computed. This precision is controlled by two key parameters:

  1. Absolute Tolerance \((\text{abstol})\)

    • Interpreted as the maximum allowable error when the solution values are near zero.
    • Ensures that numerical approximations stay within a reasonable bound, preventing physically impossible outcomes (e.g., negative concentrations) or excessive drift at small scales.
    • For instance, an absolute tolerance of \(10^{-6}\) means the solver attempts to keep the absolute error below \(10^{-6}\) whenever the solution magnitude is close to zero.
  2. Relative Tolerance \((\text{reltol})\)

    • Enforces the number of correct digits throughout the simulation, effectively controlling error relative to the current scale of the solution.
    • For example, a relative tolerance of \(10^{-3}\) implies the solver aims for three correct decimal places (i.e., the solution is accurate to within 0.1% of its current magnitude).
    • As the solution grows or shrinks, the solver adjusts its time-step size and internal computations to maintain this relative accuracy.

Sometimes decreasing tolerances can help to reduce numerical problems, e.g. to keep solutions non-negative that are mathematically guaranteed to be non-negative. Additionally, the choice of tolerances can be motivated by the application of the numerical solution: For plotting a less accurate solution, and hence larger tolerances, might be tolerable, whereas typically for model fitting a more accurate solution, and hence smaller tolerances, are beneficial.

This can be demonstrated when fitting the Warfarin model with an example dataset: Optimization fails with large tolerances of 1e-3 (relative) and 1e-6 (absolute)

fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; reltol = 1e-3, abstol = 1e-6),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     3.004020e+06     5.745853e+06
 * time: 0.029724836349487305
     1     4.816669e+05     8.316173e+05
 * time: 2.0062570571899414
     2     3.598373e+05     6.060150e+05
 * time: 2.7980690002441406
     3     1.682124e+05     2.709072e+05
 * time: 3.5151119232177734
     4     9.168914e+04     1.446832e+05
 * time: 4.313179016113281
     5     4.791138e+04     6.671463e+04
 * time: 5.025882959365845
     6     2.909104e+04     3.426663e+04
 * time: 5.763444900512695
     7     1.843737e+04     1.678953e+04
 * time: 6.530328035354614
     8     1.282447e+04     1.021463e+04
 * time: 7.219933032989502
     9     9.723361e+03     9.291650e+03
 * time: 7.991725921630859
    10     7.635613e+03     8.325311e+03
 * time: 8.663815021514893
    11     6.243129e+03     7.337397e+03
 * time: 9.362900018692017
    12     5.279957e+03     6.304542e+03
 * time: 10.008405923843384
    13     4.422123e+03     5.001962e+03
 * time: 10.682711839675903
    14     3.744748e+03     3.550400e+03
 * time: 11.273859024047852
    15     3.409674e+03     2.461547e+03
 * time: 11.843358039855957
    16     3.312049e+03     1.856430e+03
 * time: 12.481034994125366
    17     3.300559e+03     1.648439e+03
 * time: 13.16940188407898
    18     3.299489e+03     1.599460e+03
 * time: 13.724724054336548
    19     3.298436e+03     1.567520e+03
 * time: 14.305112838745117
    20     3.294921e+03     1.501363e+03
 * time: 14.873414039611816
    21     3.286641e+03     1.407664e+03
 * time: 15.440361976623535
    22     3.264637e+03     1.259077e+03
 * time: 15.997187852859497
    23     3.209594e+03     1.043534e+03
 * time: 16.617003917694092
    24     3.072117e+03     7.454677e+02
 * time: 17.164456844329834
    25     2.739794e+03     3.818063e+02
 * time: 17.715576887130737
    26     1.949883e+03     2.246644e+02
 * time: 18.329725980758667
    27     1.722434e+03     2.224562e+02
 * time: 19.85228204727173
    28     1.479355e+03     2.042452e+02
 * time: 21.790685892105103
    29     1.310583e+03     1.531868e+02
 * time: 24.28032684326172
    30     1.224675e+03     1.756826e+02
 * time: 24.87396788597107
    31     1.204841e+03     2.738440e+02
 * time: 25.3345148563385
    32     1.196432e+03     2.403217e+02
 * time: 25.86268186569214
    33     1.189131e+03     2.301186e+02
 * time: 26.33276891708374
    34     1.177881e+03     2.083030e+02
 * time: 26.88728904724121
    35     1.176989e+03     2.201993e+02
 * time: 27.392173051834106
    36     1.176899e+03     2.230826e+02
 * time: 27.94138789176941
    37     1.176882e+03     2.233531e+02
 * time: 28.405926942825317
    38     1.176795e+03     2.238567e+02
 * time: 28.92625904083252
    39     1.176609e+03     2.236914e+02
 * time: 29.392478942871094
    40     1.176090e+03     2.210941e+02
 * time: 29.949235916137695
    41     1.174841e+03     2.112741e+02
 * time: 30.45778799057007
    42     1.171973e+03     1.825298e+02
 * time: 30.994550943374634
    43     1.166737e+03     1.210559e+02
 * time: 31.48364496231079
    44     1.160698e+03     4.450697e+01
 * time: 32.02490592002869
    45     1.157926e+03     6.411275e+01
 * time: 32.5980749130249
    46     1.157537e+03     6.121762e+01
 * time: 33.152344942092896
    47     1.157520e+03     5.867223e+01
 * time: 33.65998697280884
    48     1.157519e+03     5.814523e+01
 * time: 34.27387595176697
    49     1.157515e+03     5.686007e+01
 * time: 34.793344020843506
    50     1.157505e+03     5.497418e+01
 * time: 35.33447194099426
    51     1.157477e+03     5.129724e+01
 * time: 35.83544898033142
    52     1.157408e+03     4.463171e+01
 * time: 36.37207102775574
    53     1.157239e+03     3.279117e+01
 * time: 36.88265299797058
    54     1.156869e+03     3.681526e+01
 * time: 37.42654490470886
    55     1.156230e+03     3.198754e+01
 * time: 37.93988800048828
    56     1.155563e+03     4.576438e+01
 * time: 38.478617906570435
    57     1.155264e+03     4.832159e+01
 * time: 38.987661838531494
    58     1.155212e+03     4.052175e+01
 * time: 39.51592397689819
    59     1.155207e+03     3.797125e+01
 * time: 40.01277804374695
    60     1.155206e+03     3.778452e+01
 * time: 40.53997993469238
    61     1.155201e+03     3.723003e+01
 * time: 41.04446482658386
    62     1.155190e+03     3.642375e+01
 * time: 41.6595458984375
    63     1.155159e+03     3.487535e+01
 * time: 42.16261386871338
    64     1.155079e+03     3.209060e+01
 * time: 42.68754482269287
    65     1.154875e+03     2.670227e+01
 * time: 43.19315695762634
    66     1.154375e+03     2.366165e+01
 * time: 43.716959953308105
    67     1.153278e+03     5.499525e+01
 * time: 44.2250440120697
    68     1.151457e+03     7.903833e+01
 * time: 44.738083839416504
    69     1.149737e+03     6.771227e+01
 * time: 45.2470338344574
    70     1.148983e+03     5.387708e+01
 * time: 45.8035728931427
    71     1.148872e+03     4.683950e+01
 * time: 46.27865791320801
    72     1.148868e+03     4.512036e+01
 * time: 46.763136863708496
    73     1.148866e+03     4.489706e+01
 * time: 47.21384882926941
    74     1.148857e+03     4.426314e+01
 * time: 47.66077494621277
    75     1.148838e+03     4.321161e+01
 * time: 48.09964990615845
    76     1.148785e+03     4.095232e+01
 * time: 48.545389890670776
    77     1.148652e+03     3.780572e+01
 * time: 48.98205304145813
    78     1.148325e+03     3.474239e+01
 * time: 49.487318992614746
    79     1.147599e+03     3.678957e+01
 * time: 50.002771854400635
    80     1.146330e+03     5.134277e+01
 * time: 50.52331900596619
    81     1.144979e+03     6.444063e+01
 * time: 51.068039894104004
    82     1.144325e+03     7.574751e+01
 * time: 51.59901189804077
    83     1.144210e+03     6.968296e+01
 * time: 52.1621458530426
    84     1.144204e+03     6.648234e+01
 * time: 52.612109899520874
    85     1.144202e+03     6.560999e+01
 * time: 53.1344108581543
    86     1.144194e+03     6.305667e+01
 * time: 53.593031883239746
    87     1.144176e+03     5.956959e+01
 * time: 54.115488052368164
    88     1.144127e+03     5.307828e+01
 * time: 54.56674885749817
    89     1.144005e+03     4.218776e+01
 * time: 55.080068826675415
    90     1.143708e+03     4.120261e+01
 * time: 55.55855894088745
    91     1.143074e+03     3.675516e+01
 * time: 56.10743999481201
    92     1.142052e+03     3.855439e+01
 * time: 56.60622000694275
    93     1.141101e+03     5.193197e+01
 * time: 57.165180921554565
    94     1.140701e+03     4.606858e+01
 * time: 57.64548397064209
    95     1.140646e+03     4.857736e+01
 * time: 58.179245948791504
    96     1.140643e+03     4.817596e+01
 * time: 58.65185904502869
    97     1.140642e+03     4.793197e+01
 * time: 59.17725586891174
    98     1.140637e+03     4.723245e+01
 * time: 59.653767824172974
    99     1.140626e+03     4.619695e+01
 * time: 60.15275192260742
   100     1.140597e+03     4.418761e+01
 * time: 60.605921030044556
   101     1.140523e+03     4.050986e+01
 * time: 61.09307098388672
   102     1.140334e+03     3.335334e+01
 * time: 61.5455379486084
   103     1.139892e+03     2.982120e+01
 * time: 62.04730486869812
   104     1.138994e+03     4.360237e+01
 * time: 62.49710488319397
   105     1.137687e+03     5.246892e+01
 * time: 62.99178695678711
   106     1.136679e+03     4.825586e+01
 * time: 63.423651933670044
   107     1.136391e+03     4.467499e+01
 * time: 63.93027091026306
   108     1.136364e+03     3.961591e+01
 * time: 64.38046884536743
   109     1.136362e+03     3.951402e+01
 * time: 64.85402989387512
   110     1.136360e+03     3.943358e+01
 * time: 65.28286099433899
   111     1.136355e+03     3.923965e+01
 * time: 65.75208902359009
   112     1.136344e+03     3.894674e+01
 * time: 66.19487500190735
   113     1.136314e+03     3.841518e+01
 * time: 66.67323303222656
   114     1.136235e+03     3.748824e+01
 * time: 67.12131404876709
   115     1.136029e+03     3.577210e+01
 * time: 67.60785794258118
   116     1.135499e+03     3.252332e+01
 * time: 68.0646140575409
   117     1.134182e+03     2.630844e+01
 * time: 68.56378984451294
   118     1.131238e+03     4.869163e+01
 * time: 69.03646898269653
   119     1.126345e+03     6.537856e+01
 * time: 69.53587603569031
   120     1.121879e+03     5.123373e+01
 * time: 70.03179693222046
   121     1.119645e+03     2.457316e+01
 * time: 70.55405592918396
   122     1.119031e+03     2.124721e+01
 * time: 71.06296491622925
   123     1.118958e+03     1.857508e+01
 * time: 71.5822069644928
   124     1.118953e+03     1.896595e+01
 * time: 72.07880282402039
   125     1.118952e+03     1.849021e+01
 * time: 72.62868785858154
   126     1.118952e+03     1.852941e+01
 * time: 73.13885593414307
   127     1.118951e+03     1.856812e+01
 * time: 73.64048886299133
   128     1.118951e+03     1.862403e+01
 * time: 74.13744902610779
   129     1.118949e+03     1.871484e+01
 * time: 74.6356680393219
   130     1.118946e+03     1.886332e+01
 * time: 75.11932587623596
   131     1.118939e+03     1.910217e+01
 * time: 75.61092400550842
   132     1.118922e+03     1.948050e+01
 * time: 76.09845495223999
   133     1.118879e+03     2.006903e+01
 * time: 76.5951418876648
   134     1.118769e+03     2.097106e+01
 * time: 77.05900192260742
   135     1.118486e+03     2.231801e+01
 * time: 77.5318808555603
   136     1.117755e+03     3.049039e+01
 * time: 77.99631595611572
   137     1.115883e+03     5.345487e+01
 * time: 78.48412084579468
   138     1.111258e+03     8.489274e+01
 * time: 78.97799301147461
   139     1.101149e+03     7.643749e+01
 * time: 79.55168795585632
   140     1.090612e+03     5.802659e+01
 * time: 80.18081188201904
   141     1.087971e+03     5.776554e+01
 * time: 80.75567889213562
   142     1.087205e+03     1.148234e+02
 * time: 81.4542829990387
   143     1.085558e+03     4.724872e+01
 * time: 82.10414385795593
   144     1.085271e+03     2.018848e+01
 * time: 82.96370005607605
   145     1.085124e+03     2.004201e+01
 * time: 83.56954789161682
   146     1.085089e+03     2.025616e+01
 * time: 84.21434593200684
   147     1.085081e+03     2.049682e+01
 * time: 84.83074498176575
   148     1.085073e+03     2.086075e+01
 * time: 85.48445296287537
   149     1.085070e+03     2.095706e+01
 * time: 86.09431004524231
   150     1.085063e+03     2.094978e+01
 * time: 86.7024998664856
   151     1.085062e+03     2.093855e+01
 * time: 87.38792896270752
   152     1.085062e+03     2.093439e+01
 * time: 88.07346892356873
   153     1.085062e+03     2.093335e+01
 * time: 88.83808398246765
   154     1.085062e+03     2.093252e+01
 * time: 89.57580494880676
   155     1.085062e+03     2.093227e+01
 * time: 90.36978197097778
   156     1.085062e+03     2.093208e+01
 * time: 91.20789098739624
   157     1.085062e+03     2.093187e+01
 * time: 92.02035903930664
   158     1.085062e+03     2.093181e+01
 * time: 92.83537697792053
   159     1.085062e+03     2.093175e+01
 * time: 93.66535997390747
   160     1.085062e+03     2.093172e+01
 * time: 94.53683090209961
   161     1.085062e+03     2.046653e+01
 * time: 95.11549186706543
   162     1.085061e+03     2.045920e+01
 * time: 95.7680070400238
   163     1.085056e+03     2.021786e+01
 * time: 96.35349702835083
   164     1.085055e+03     2.018381e+01
 * time: 96.91465783119202
   165     1.085043e+03     1.997498e+01
 * time: 97.48475885391235
   166     1.085029e+03     1.961695e+01
 * time: 98.00829100608826
   167     1.084997e+03     1.929634e+01
 * time: 98.56966996192932
   168     1.084881e+03     1.855033e+01
 * time: 99.08399391174316
   169     1.084633e+03     1.799616e+01
 * time: 99.63483095169067
   170     1.083831e+03     1.856893e+01
 * time: 100.15179085731506
   171     1.081240e+03     2.069096e+01
 * time: 100.71300983428955
   172     1.072465e+03     3.262436e+01
 * time: 101.32411885261536
   173     1.072446e+03     6.686587e+01
 * time: 101.92538404464722
   174     1.068493e+03     7.417042e+01
 * time: 102.5943808555603
   175     1.067469e+03     7.109936e+01
 * time: 103.26672291755676
   176     1.065854e+03     2.774005e+01
 * time: 103.92106103897095
   177     1.065215e+03     1.328875e+01
 * time: 104.5223159790039
   178     1.065128e+03     1.390400e+01
 * time: 105.15694189071655
   179     1.065120e+03     1.383172e+01
 * time: 105.742928981781
   180     1.065120e+03     1.384995e+01
 * time: 106.36983585357666
FittedPumasModel

Successful minimization:                     false

Likelihood approximation:                     FOCE
Likelihood Optimizer:                         BFGS
Dynamical system type:               Nonlinear ODE
Solver(s):(OrdinaryDiffEq.Vern7,OrdinaryDiffEq.Rodas5P)

Log-likelihood value:                   -1065.1201
Number of subjects:                             31
Number of parameters:         Fixed      Optimized
                                  0             18
Observation records:         Active        Missing
    conc:                       239             47
    pca:                        225             61
    Total:                      464            108

-------------------------
              Estimate
-------------------------
pop_CL         0.13142
pop_V          7.9732
pop_tabs       0.57389
pop_lag        0.83565
pop_e0        96.508
pop_emax      -1.0982
pop_c50        1.7031
pop_tover     14.804
pk_Ω₁,₁        0.078665
pk_Ω₂,₂        0.031456
pk_Ω₃,₃        0.083202
pd_Ω₁,₁        0.00316
pd_Ω₂,₂        0.0023756
pd_Ω₃,₃        0.025823
pd_Ω₄,₄        0.013808
σ_prop         0.013696
σ_add          0.84173
σ_fx           3.3937
-------------------------

but succeeds with lower tolerances of 1e-8 (relative) and 1e-12 (absolute):

fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     2.998927e+06     5.740466e+06
 * time: 7.009506225585938e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.4213109016418457
     2     3.595691e+05     6.035843e+05
 * time: 2.776289939880371
     3     1.691071e+05     2.702705e+05
 * time: 3.995556116104126
     4     9.193152e+04     1.451040e+05
 * time: 5.28485107421875
     5     4.768539e+04     6.630202e+04
 * time: 6.495600938796997
     6     2.904205e+04     3.416919e+04
 * time: 7.938838958740234
     7     1.840153e+04     1.671742e+04
 * time: 9.321347951889038
     8     1.280989e+04     1.022268e+04
 * time: 10.629220008850098
     9     9.713287e+03     9.298485e+03
 * time: 12.010786056518555
    10     7.628127e+03     8.330656e+03
 * time: 13.268932104110718
    11     6.236278e+03     7.340367e+03
 * time: 14.551939010620117
    12     5.271958e+03     6.303773e+03
 * time: 15.7865629196167
    13     4.411047e+03     4.994130e+03
 * time: 17.045713901519775
    14     3.732936e+03     3.538184e+03
 * time: 18.29279088973999
    15     3.399273e+03     2.450045e+03
 * time: 19.517412900924683
    16     3.302987e+03     1.848476e+03
 * time: 20.68213391304016
    17     3.291788e+03     1.642864e+03
 * time: 21.837488889694214
    18     3.290743e+03     1.594765e+03
 * time: 22.97154998779297
    19     3.289687e+03     1.562641e+03
 * time: 24.121416091918945
    20     3.286186e+03     1.496632e+03
 * time: 25.237525939941406
    21     3.277917e+03     1.402865e+03
 * time: 26.383261919021606
    22     3.255959e+03     1.254297e+03
 * time: 27.5228750705719
    23     3.200980e+03     1.038621e+03
 * time: 28.67260503768921
    24     3.063521e+03     7.403605e+02
 * time: 29.803741931915283
    25     2.730316e+03     3.768701e+02
 * time: 30.854473114013672
    26     1.934186e+03     2.246121e+02
 * time: 31.976321935653687
    27     1.704399e+03     2.219826e+02
 * time: 36.09633111953735
    28     1.471183e+03     2.025826e+02
 * time: 43.09261894226074
    29     1.316789e+03     1.554714e+02
 * time: 48.958149909973145
    30     1.226363e+03     1.775666e+02
 * time: 50.08729696273804
    31     1.206626e+03     2.780980e+02
 * time: 51.15478301048279
    32     1.197830e+03     2.430550e+02
 * time: 52.148098945617676
    33     1.190342e+03     2.327905e+02
 * time: 53.20940709114075
    34     1.178783e+03     2.105809e+02
 * time: 54.22853112220764
    35     1.177853e+03     2.228585e+02
 * time: 55.30527305603027
    36     1.177763e+03     2.256116e+02
 * time: 56.301328897476196
    37     1.177745e+03     2.259359e+02
 * time: 57.291074991226196
    38     1.177670e+03     2.264633e+02
 * time: 58.263449907302856
    39     1.177500e+03     2.264574e+02
 * time: 59.31709003448486
    40     1.177033e+03     2.244136e+02
 * time: 60.43332505226135
    41     1.175894e+03     2.160226e+02
 * time: 61.64549708366394
    42     1.173240e+03     1.905912e+02
 * time: 63.849257946014404
    43     1.168189e+03     1.332783e+02
 * time: 65.26796793937683
    44     1.161835e+03     5.289952e+01
 * time: 66.54442405700684
    45     1.158453e+03     6.595387e+01
 * time: 67.79223203659058
    46     1.157897e+03     6.411419e+01
 * time: 68.93668389320374
    47     1.157870e+03     6.098509e+01
 * time: 70.15670895576477
    48     1.157868e+03     6.032325e+01
 * time: 71.31102108955383
    49     1.157864e+03     5.920812e+01
 * time: 72.50489807128906
    50     1.157856e+03     5.740225e+01
 * time: 73.58168911933899
    51     1.157832e+03     5.406795e+01
 * time: 74.67741107940674
    52     1.157774e+03     4.797879e+01
 * time: 75.70085906982422
    53     1.157628e+03     3.658732e+01
 * time: 76.75829911231995
    54     1.157299e+03     3.707874e+01
 * time: 77.78539896011353
    55     1.156690e+03     3.452947e+01
 * time: 78.79574298858643
    56     1.155959e+03     4.269004e+01
 * time: 79.77993893623352
    57     1.155557e+03     5.000322e+01
 * time: 80.77716994285583
    58     1.155472e+03     4.219349e+01
 * time: 81.76612710952759
    59     1.155463e+03     3.895049e+01
 * time: 82.80450701713562
    60     1.155462e+03     3.872099e+01
 * time: 83.78659796714783
    61     1.155458e+03     3.823491e+01
 * time: 84.76249599456787
    62     1.155450e+03     3.747370e+01
 * time: 85.73295092582703
    63     1.155426e+03     3.608648e+01
 * time: 86.702064037323
    64     1.155365e+03     3.361118e+01
 * time: 87.68594408035278
    65     1.155207e+03     2.894678e+01
 * time: 88.69814491271973
    66     1.154814e+03     2.504284e+01
 * time: 89.66562390327454
    67     1.153910e+03     4.541126e+01
 * time: 90.61883306503296
    68     1.152230e+03     7.426044e+01
 * time: 91.54212093353271
    69     1.150286e+03     7.473044e+01
 * time: 92.47210192680359
    70     1.149188e+03     5.605967e+01
 * time: 93.39829397201538
    71     1.148944e+03     4.801123e+01
 * time: 94.35852193832397
    72     1.148931e+03     4.659478e+01
 * time: 95.25754404067993
    73     1.148929e+03     4.635180e+01
 * time: 96.16334390640259
    74     1.148922e+03     4.568579e+01
 * time: 97.05272197723389
    75     1.148907e+03     4.466128e+01
 * time: 97.96738290786743
    76     1.148864e+03     4.249742e+01
 * time: 98.86467599868774
    77     1.148758e+03     3.824840e+01
 * time: 99.80262207984924
    78     1.148490e+03     3.552459e+01
 * time: 100.74380207061768
    79     1.147881e+03     3.038181e+01
 * time: 101.72020411491394
    80     1.146736e+03     4.467281e+01
 * time: 102.68127107620239
    81     1.145347e+03     5.929158e+01
 * time: 103.59850001335144
    82     1.144535e+03     7.572784e+01
 * time: 104.51590609550476
    83     1.144354e+03     7.034986e+01
 * time: 105.4471960067749
    84     1.144341e+03     6.591584e+01
 * time: 106.38865089416504
    85     1.144340e+03     6.496353e+01
 * time: 107.4462399482727
    86     1.144333e+03     6.268267e+01
 * time: 108.44511008262634
    87     1.144320e+03     5.950121e+01
 * time: 109.56558108329773
    88     1.144281e+03     5.368613e+01
 * time: 110.68823194503784
    89     1.144186e+03     4.393956e+01
 * time: 111.71031594276428
    90     1.143949e+03     4.102324e+01
 * time: 112.74802303314209
    91     1.143423e+03     3.752005e+01
 * time: 113.81203007698059
    92     1.142497e+03     3.278884e+01
 * time: 114.84415102005005
    93     1.141489e+03     5.217814e+01
 * time: 115.90482306480408
    94     1.140969e+03     4.310032e+01
 * time: 116.92359709739685
    95     1.140872e+03     4.759545e+01
 * time: 117.95670008659363
    96     1.140867e+03     4.727034e+01
 * time: 119.00169801712036
    97     1.140866e+03     4.704365e+01
 * time: 119.97153997421265
    98     1.140862e+03     4.637923e+01
 * time: 121.02337098121643
    99     1.140853e+03     4.542582e+01
 * time: 121.97108006477356
   100     1.140829e+03     4.357378e+01
 * time: 123.01005291938782
   101     1.140767e+03     4.023060e+01
 * time: 123.98621702194214
   102     1.140610e+03     3.376323e+01
 * time: 125.02360200881958
   103     1.140237e+03     2.857028e+01
 * time: 126.00293207168579
   104     1.139451e+03     3.895699e+01
 * time: 127.08703994750977
   105     1.138223e+03     5.064036e+01
 * time: 128.0933859348297
   106     1.137140e+03     4.763314e+01
 * time: 129.15878796577454
   107     1.136763e+03     4.604968e+01
 * time: 130.15141892433167
   108     1.136720e+03     3.993479e+01
 * time: 131.1869010925293
   109     1.136717e+03     3.890565e+01
 * time: 132.1346640586853
   110     1.136716e+03     3.883645e+01
 * time: 133.10726690292358
   111     1.136712e+03     3.865517e+01
 * time: 134.02548003196716
   112     1.136702e+03     3.838833e+01
 * time: 134.99683690071106
   113     1.136674e+03     3.789981e+01
 * time: 135.96853709220886
   114     1.136603e+03     3.705558e+01
 * time: 137.00783705711365
   115     1.136418e+03     3.549940e+01
 * time: 138.00457000732422
   116     1.135940e+03     3.257136e+01
 * time: 139.12191200256348
   117     1.134743e+03     2.697248e+01
 * time: 140.13566899299622
   118     1.132014e+03     4.863416e+01
 * time: 141.18758606910706
   119     1.127203e+03     6.901202e+01
 * time: 142.22617602348328
   120     1.122363e+03     5.794311e+01
 * time: 143.3027160167694
   121     1.119759e+03     2.906766e+01
 * time: 144.32950496673584
   122     1.118972e+03     2.137843e+01
 * time: 145.37162399291992
   123     1.118870e+03     1.909639e+01
 * time: 146.33086705207825
   124     1.118863e+03     1.887024e+01
 * time: 147.38123989105225
   125     1.118862e+03     1.852691e+01
 * time: 148.37093210220337
   126     1.118861e+03     1.864783e+01
 * time: 149.40150809288025
   127     1.118861e+03     1.862870e+01
 * time: 150.3906090259552
   128     1.118860e+03     1.866000e+01
 * time: 151.44108605384827
   129     1.118859e+03     1.869625e+01
 * time: 152.49187302589417
   130     1.118856e+03     1.878717e+01
 * time: 153.5887908935547
   131     1.118850e+03     1.894283e+01
 * time: 154.61494708061218
   132     1.118834e+03     1.922180e+01
 * time: 155.6342489719391
   133     1.118794e+03     1.968060e+01
 * time: 156.62037301063538
   134     1.118693e+03     2.042353e+01
 * time: 157.6371009349823
   135     1.118431e+03     2.156998e+01
 * time: 158.6648759841919
   136     1.117751e+03     2.596600e+01
 * time: 159.7586851119995
   137     1.116007e+03     4.716393e+01
 * time: 160.80476689338684
   138     1.111677e+03     7.744992e+01
 * time: 161.82704901695251
   139     1.102024e+03     7.496268e+01
 * time: 162.8394649028778
   140     1.090649e+03     6.246756e+01
 * time: 164.08347702026367
   141     1.087843e+03     8.941834e+01
 * time: 165.37228298187256
   142     1.087400e+03     1.188539e+02
 * time: 166.88403296470642
   143     1.085563e+03     6.082023e+01
 * time: 168.2052550315857
   144     1.085185e+03     1.947062e+01
 * time: 169.57203888893127
   145     1.085039e+03     1.958695e+01
 * time: 170.81839799880981
   146     1.084989e+03     1.989918e+01
 * time: 171.99419498443604
   147     1.084983e+03     2.009395e+01
 * time: 173.2392599582672
   148     1.084973e+03     2.065564e+01
 * time: 174.31207609176636
   149     1.084971e+03     2.080010e+01
 * time: 175.51333904266357
   150     1.084971e+03     2.084232e+01
 * time: 176.64626693725586
   151     1.084970e+03     2.084506e+01
 * time: 177.8656189441681
   152     1.084970e+03     2.084321e+01
 * time: 178.9584460258484
   153     1.084968e+03     2.084737e+01
 * time: 180.02079701423645
   154     1.084963e+03     2.088045e+01
 * time: 181.01086497306824
   155     1.084954e+03     2.098376e+01
 * time: 181.9968249797821
   156     1.084935e+03     2.121619e+01
 * time: 183.01522207260132
   157     1.084904e+03     2.160158e+01
 * time: 184.132719039917
   158     1.084864e+03     2.208328e+01
 * time: 185.2320749759674
   159     1.084807e+03     2.260004e+01
 * time: 186.35443210601807
   160     1.084700e+03     2.319870e+01
 * time: 187.4541790485382
   161     1.084457e+03     2.382124e+01
 * time: 188.5581018924713
   162     1.083882e+03     2.406095e+01
 * time: 189.6378779411316
   163     1.082589e+03     2.849219e+01
 * time: 190.74591708183289
   164     1.081660e+03     3.379635e+01
 * time: 192.06736993789673
   165     1.078576e+03     4.983497e+01
 * time: 193.42579412460327
   166     1.075004e+03     6.870527e+01
 * time: 194.68409299850464
   167     1.069377e+03     6.309176e+01
 * time: 195.84553909301758
   168     1.067978e+03     3.811944e+01
 * time: 197.01203894615173
   169     1.065604e+03     1.825143e+01
 * time: 198.17500400543213
   170     1.065251e+03     1.739631e+01
 * time: 199.45450711250305
   171     1.065088e+03     1.384781e+01
 * time: 200.89711499214172
   172     1.065086e+03     1.382386e+01
 * time: 202.62313604354858
   173     1.065086e+03     1.383866e+01
 * time: 204.0244688987732
   174     1.065086e+03     1.384608e+01
 * time: 205.51548099517822
   175     1.065086e+03     1.385678e+01
 * time: 206.97889709472656
   176     1.065084e+03     1.387273e+01
 * time: 208.54461908340454
   177     1.065081e+03     1.389858e+01
 * time: 209.89624404907227
   178     1.065073e+03     1.393897e+01
 * time: 211.27133202552795
   179     1.065052e+03     1.400288e+01
 * time: 212.59741806983948
   180     1.064997e+03     1.842052e+01
 * time: 213.94041109085083
   181     1.064853e+03     3.031555e+01
 * time: 215.29034805297852
   182     1.064477e+03     4.894991e+01
 * time: 216.6684238910675
   183     1.063505e+03     7.692533e+01
 * time: 218.0107879638672
   184     1.061069e+03     1.122192e+02
 * time: 219.41456508636475
   185     1.056257e+03     1.328367e+02
 * time: 220.8858940601349
   186     1.052013e+03     1.408303e+02
 * time: 222.45825791358948
   187     1.047713e+03     1.506293e+02
 * time: 224.02736711502075
   188     1.039359e+03     9.039176e+01
 * time: 225.583251953125
   189     1.037334e+03     8.945366e+01
 * time: 227.25305700302124
   190     1.035766e+03     2.509488e+01
 * time: 228.87203788757324
   191     1.035510e+03     5.323250e+00
 * time: 230.59712290763855
   192     1.035401e+03     3.326584e+00
 * time: 232.1108410358429
   193     1.035202e+03     1.011867e+01
 * time: 233.6306529045105
   194     1.035130e+03     9.089057e+00
 * time: 235.06331706047058
   195     1.035055e+03     3.941605e+00
 * time: 236.52071404457092
   196     1.035048e+03     3.160942e+00
 * time: 237.91156697273254
   197     1.035046e+03     3.164670e+00
 * time: 239.30720496177673
   198     1.035046e+03     3.160486e+00
 * time: 240.65007495880127
   199     1.035046e+03     3.159428e+00
 * time: 242.02116703987122
   200     1.035046e+03     3.159138e+00
 * time: 243.35013008117676
   201     1.035046e+03     3.158158e+00
 * time: 244.71428894996643
   202     1.035046e+03     3.156875e+00
 * time: 246.04402709007263
   203     1.035046e+03     3.154403e+00
 * time: 247.45249009132385
   204     1.035045e+03     3.150100e+00
 * time: 248.7798399925232
   205     1.035043e+03     3.141854e+00
 * time: 250.1157341003418
   206     1.035038e+03     3.125468e+00
 * time: 251.42922711372375
   207     1.035025e+03     3.887763e+00
 * time: 252.8229501247406
   208     1.034991e+03     6.349435e+00
 * time: 254.16261291503906
   209     1.034902e+03     1.025429e+01
 * time: 255.5246570110321
   210     1.034678e+03     1.614807e+01
 * time: 256.87574791908264
   211     1.034143e+03     2.389258e+01
 * time: 258.2801389694214
   212     1.033018e+03     3.065325e+01
 * time: 259.64242911338806
   213     1.031157e+03     2.988569e+01
 * time: 261.04354906082153
   214     1.029913e+03     2.000449e+01
 * time: 262.48559498786926
   215     1.029264e+03     1.806624e+01
 * time: 263.85137605667114
   216     1.028739e+03     1.876674e+01
 * time: 265.3217360973358
   217     1.028529e+03     1.788313e+01
 * time: 266.82394909858704
   218     1.028407e+03     1.869928e+01
 * time: 268.2471730709076
   219     1.028318e+03     2.103440e+01
 * time: 269.6408739089966
   220     1.028259e+03     2.278116e+01
 * time: 271.07591104507446
   221     1.028193e+03     2.168720e+01
 * time: 272.30632495880127
   222     1.028177e+03     2.174690e+01
 * time: 273.5475010871887
   223     1.028175e+03     2.195554e+01
 * time: 274.8123450279236
   224     1.028175e+03     2.193442e+01
 * time: 276.1940829753876
   225     1.028174e+03     2.202037e+01
 * time: 277.61642599105835
   226     1.028174e+03     2.201903e+01
 * time: 278.9976439476013
   227     1.028173e+03     2.200004e+01
 * time: 280.3721480369568
   228     1.028171e+03     2.197102e+01
 * time: 281.7960729598999
   229     1.028164e+03     2.191369e+01
 * time: 283.19824290275574
   230     1.028148e+03     2.181626e+01
 * time: 284.7013099193573
   231     1.028106e+03     2.163664e+01
 * time: 286.11368894577026
   232     1.027999e+03     3.038649e+01
 * time: 287.53586411476135
   233     1.027721e+03     5.035972e+01
 * time: 288.9657769203186
   234     1.027021e+03     8.114702e+01
 * time: 290.3670299053192
   235     1.025404e+03     1.233505e+02
 * time: 291.7490770816803
   236     1.022343e+03     1.646448e+02
 * time: 293.06224489212036
   237     1.017556e+03     1.679173e+02
 * time: 294.3616690635681
   238     1.013261e+03     5.600035e+01
 * time: 295.6697270870209
   239     1.012953e+03     2.691504e+01
 * time: 297.0807249546051
   240     1.012758e+03     1.329779e+01
 * time: 298.29811811447144
   241     1.012704e+03     2.847118e+00
 * time: 299.6186349391937
   242     1.012699e+03     1.333414e+00
 * time: 300.8747088909149
   243     1.012697e+03     9.979337e-01
 * time: 302.2153251171112
   244     1.012697e+03     9.969545e-01
 * time: 303.4618248939514
   245     1.012697e+03     9.971341e-01
 * time: 304.70461893081665
   246     1.012697e+03     9.971972e-01
 * time: 305.929505109787
   247     1.012697e+03     9.972237e-01
 * time: 307.2168290615082
   248     1.012697e+03     9.972927e-01
 * time: 308.45721912384033
   249     1.012697e+03     9.973804e-01
 * time: 309.70267605781555
   250     1.012697e+03     9.975286e-01
 * time: 310.876149892807
   251     1.012697e+03     9.977461e-01
 * time: 312.1103711128235
   252     1.012697e+03     9.980635e-01
 * time: 313.36023592948914
   253     1.012697e+03     9.984743e-01
 * time: 314.7837669849396
   254     1.012696e+03     9.988765e-01
 * time: 316.4778079986572
   255     1.012694e+03     9.988363e-01
 * time: 319.1981439590454
   256     1.012690e+03     1.474525e+00
 * time: 320.9471130371094
   257     1.012678e+03     2.326520e+00
 * time: 322.6200120449066
   258     1.012649e+03     3.510529e+00
 * time: 324.1880979537964
   259     1.012590e+03     4.737310e+00
 * time: 325.68547010421753
   260     1.012497e+03     4.897808e+00
 * time: 327.0984148979187
   261     1.012416e+03     2.875911e+00
 * time: 328.5509099960327
   262     1.012389e+03     9.301211e-01
 * time: 329.95952892303467
   263     1.012386e+03     8.804644e-01
 * time: 331.381019115448
   264     1.012386e+03     8.547350e-01
 * time: 332.72533202171326
   265     1.012386e+03     8.516621e-01
 * time: 334.0848801136017
   266     1.012386e+03     8.511780e-01
 * time: 335.4026880264282
   267     1.012386e+03     8.489977e-01
 * time: 336.77585911750793
   268     1.012386e+03     8.463566e-01
 * time: 338.05095291137695
   269     1.012386e+03     8.415231e-01
 * time: 339.3780241012573
   270     1.012386e+03     8.340037e-01
 * time: 340.6795370578766
   271     1.012386e+03     8.215489e-01
 * time: 342.09105801582336
   272     1.012386e+03     8.013257e-01
 * time: 343.37320494651794
   273     1.012385e+03     7.680260e-01
 * time: 344.6681890487671
   274     1.012383e+03     7.407512e-01
 * time: 345.94519805908203
   275     1.012377e+03     7.469071e-01
 * time: 347.29879903793335
   276     1.012364e+03     1.103988e+00
 * time: 348.59408807754517
   277     1.012330e+03     1.710981e+00
 * time: 350.00449299812317
   278     1.012257e+03     2.412723e+00
 * time: 351.2990839481354
   279     1.012125e+03     2.705165e+00
 * time: 352.59764099121094
   280     1.011948e+03     1.883597e+00
 * time: 353.83462500572205
   281     1.011809e+03     8.215973e-01
 * time: 355.0770230293274
   282     1.011775e+03     9.067203e-01
 * time: 356.23260498046875
   283     1.011774e+03     9.565073e-01
 * time: 357.503525018692
   284     1.011774e+03     9.628263e-01
 * time: 358.696918964386
   285     1.011774e+03     9.654767e-01
 * time: 359.97710609436035
   286     1.011774e+03     9.654769e-01
 * time: 361.3122639656067
   287     1.011774e+03     9.656366e-01
 * time: 362.5804350376129
   288     1.011774e+03     9.656622e-01
 * time: 363.8001799583435
   289     1.011774e+03     9.656624e-01
 * time: 365.1738450527191
   290     1.011774e+03     9.656627e-01
 * time: 366.58563208580017
   291     1.011774e+03     9.660395e-01
 * time: 367.88693594932556
   292     1.011774e+03     9.661257e-01
 * time: 369.15504002571106
   293     1.011774e+03     9.663115e-01
 * time: 370.46117091178894
   294     1.011774e+03     9.662851e-01
 * time: 371.741534948349
   295     1.011773e+03     9.654773e-01
 * time: 372.99481201171875
   296     1.011772e+03     9.623124e-01
 * time: 374.2086009979248
   297     1.011769e+03     9.521889e-01
 * time: 375.45868396759033
   298     1.011763e+03     9.239616e-01
 * time: 376.604043006897
   299     1.011746e+03     9.242954e-01
 * time: 377.8690130710602
   300     1.011711e+03     1.257052e+00
 * time: 379.07086300849915
   301     1.011651e+03     1.361278e+00
 * time: 380.26551508903503
   302     1.011591e+03     9.076916e-01
 * time: 381.40745401382446
   303     1.011568e+03     3.084752e-01
 * time: 382.57904505729675
   304     1.011565e+03     2.946404e-01
 * time: 383.83995389938354
   305     1.011565e+03     2.821273e-01
 * time: 385.1048529148102
   306     1.011565e+03     2.766260e-01
 * time: 386.4107060432434
   307     1.011565e+03     2.778010e-01
 * time: 387.734482049942
   308     1.011565e+03     2.830281e-01
 * time: 388.9585530757904
   309     1.011565e+03     2.842619e-01
 * time: 390.29695296287537
   310     1.011565e+03     2.861093e-01
 * time: 391.59561491012573
   311     1.011565e+03     2.861138e-01
 * time: 392.9492540359497
   312     1.011565e+03     2.867641e-01
 * time: 394.25069403648376
   313     1.011565e+03     2.867756e-01
 * time: 395.6198670864105
   314     1.011565e+03     2.884589e-01
 * time: 396.86465191841125
   315     1.011565e+03     2.897335e-01
 * time: 398.04496002197266
   316     1.011565e+03     2.927744e-01
 * time: 399.1425449848175
   317     1.011565e+03     2.967424e-01
 * time: 400.25126600265503
   318     1.011565e+03     3.027347e-01
 * time: 401.37266993522644
   319     1.011564e+03     3.099827e-01
 * time: 402.5756850242615
   320     1.011563e+03     3.155826e-01
 * time: 403.74416303634644
   321     1.011560e+03     3.082844e-01
 * time: 404.9813470840454
   322     1.011555e+03     2.625121e-01
 * time: 406.3092210292816
   323     1.011548e+03     2.136271e-01
 * time: 407.7033441066742
   324     1.011542e+03     1.165033e-01
 * time: 409.1061511039734
   325     1.011538e+03     3.914224e-02
 * time: 410.49344205856323
   326     1.011537e+03     3.083888e-02
 * time: 411.82163405418396
   327     1.011536e+03     2.766364e-02
 * time: 413.16252303123474
   328     1.011536e+03     1.873262e-02
 * time: 414.42528796195984
   329     1.011536e+03     5.453951e-03
 * time: 415.7236270904541
   330     1.011536e+03     4.882951e-03
 * time: 416.98046708106995
   331     1.011535e+03     4.331879e-03
 * time: 418.1123208999634
   332     1.011535e+03     2.711268e-03
 * time: 419.2297189235687
   333     1.011535e+03     4.474256e-04
 * time: 420.3970100879669
FittedPumasModel

Successful minimization:                      true

Likelihood approximation:                     FOCE
Likelihood Optimizer:                         BFGS
Dynamical system type:               Nonlinear ODE
Solver(s):(OrdinaryDiffEq.Vern7,OrdinaryDiffEq.Rodas5P)

Log-likelihood value:                   -1011.5354
Number of subjects:                             31
Number of parameters:         Fixed      Optimized
                                  0             18
Observation records:         Active        Missing
    conc:                       239             47
    pca:                        225             61
    Total:                      464            108

-------------------------
              Estimate
-------------------------
pop_CL         0.13137
pop_V          8.0255
pop_tabs       0.42635
pop_lag        0.92336
pop_e0        96.688
pop_emax      -1.0628
pop_c50        1.5016
pop_tover     14.055
pk_Ω₁,₁        0.050744
pk_Ω₂,₂        0.021666
pk_Ω₃,₃        0.97884
pd_Ω₁,₁        0.0028828
pd_Ω₂,₂        7.3726e-9
pd_Ω₃,₃        0.15184
pd_Ω₄,₄        0.011913
σ_prop         0.088292
σ_add          0.36362
σ_fx           3.5336
-------------------------
Tip

It is not recommended to decrease tolerances below 1e-14.

7.1 Changing the Algorithm

Usually, it should not be necessary to adjust the differential equation solver. If you change the solver, you should follow the guidelines in the SciML documentation that explains which solvers are the most efficient at the desired tolerance level.

For instance, if it is known that a differential equation is stiff, a stiff solver such as Rosenbrock23 at high tolerances or Rodas5P at low tolerances could be a possible alternative to the default auto-switching solver:

# Fitting with stiff solver Rodas5P at low tolerances (relative: 1e-8, absolute: 1e-12)
fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; alg = Rodas5P(), reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     2.998927e+06     5.740466e+06
 * time: 0.00011110305786132812
     1     4.804005e+05     8.280311e+05
 * time: 7.673443078994751
     2     3.595691e+05     6.035842e+05
 * time: 14.667448043823242
     3     1.691071e+05     2.702705e+05
 * time: 21.982383966445923
     4     9.193152e+04     1.451040e+05
 * time: 29.150563955307007
     5     4.768538e+04     6.630202e+04
 * time: 36.0916850566864
     6     2.904205e+04     3.416919e+04
 * time: 43.342525005340576
     7     1.840152e+04     1.671742e+04
 * time: 51.0553240776062
     8     1.280989e+04     1.022268e+04
 * time: 57.61330509185791
     9     9.713285e+03     9.298485e+03
 * time: 65.13860392570496
    10     7.628127e+03     8.330656e+03
 * time: 72.40481901168823
    11     6.236278e+03     7.340367e+03
 * time: 79.24578905105591
    12     5.271958e+03     6.303773e+03
 * time: 85.79942393302917
    13     4.411047e+03     4.994130e+03
 * time: 91.93942904472351
    14     3.732935e+03     3.538183e+03
 * time: 98.22914505004883
    15     3.399272e+03     2.450045e+03
 * time: 107.67582511901855
    16     3.302987e+03     1.848475e+03
 * time: 116.78793692588806
    17     3.291788e+03     1.642864e+03
 * time: 124.82811498641968
    18     3.290743e+03     1.594765e+03
 * time: 131.76890802383423
    19     3.289686e+03     1.562640e+03
 * time: 138.32961702346802
    20     3.286186e+03     1.496632e+03
 * time: 144.80122089385986
    21     3.277917e+03     1.402865e+03
 * time: 150.77416896820068
    22     3.255958e+03     1.254296e+03
 * time: 156.94855093955994
    23     3.200979e+03     1.038621e+03
 * time: 163.40392899513245
    24     3.063520e+03     7.403602e+02
 * time: 170.2171070575714
    25     2.730315e+03     3.768701e+02
 * time: 177.24845790863037
    26     1.934186e+03     2.246121e+02
 * time: 184.1445391178131
    27     1.704399e+03     2.219826e+02
 * time: 202.86816906929016
    28     1.471184e+03     2.025825e+02
 * time: 234.67003798484802
    29     1.316789e+03     1.554715e+02
 * time: 262.2527129650116
    30     1.226363e+03     1.775721e+02
 * time: 269.91614603996277
    31     1.206626e+03     2.781030e+02
 * time: 276.72540807724
    32     1.197830e+03     2.430603e+02
 * time: 283.01646304130554
    33     1.190342e+03     2.327966e+02
 * time: 288.7265040874481
    34     1.178784e+03     2.105878e+02
 * time: 294.6336200237274
    35     1.177853e+03     2.228653e+02
 * time: 300.4709289073944
    36     1.177764e+03     2.256182e+02
 * time: 306.3016629219055
    37     1.177746e+03     2.259425e+02
 * time: 312.17793107032776
    38     1.177670e+03     2.264699e+02
 * time: 317.8813691139221
    39     1.177500e+03     2.264641e+02
 * time: 324.4756679534912
    40     1.177034e+03     2.244207e+02
 * time: 331.4531469345093
    41     1.175895e+03     2.160307e+02
 * time: 338.1889669895172
    42     1.173241e+03     1.906017e+02
 * time: 345.25024604797363
    43     1.168190e+03     1.332917e+02
 * time: 352.0649869441986
    44     1.161836e+03     5.291022e+01
 * time: 358.8696930408478
    45     1.158453e+03     6.595471e+01
 * time: 365.6422998905182
    46     1.157897e+03     6.411640e+01
 * time: 371.92811393737793
    47     1.157870e+03     6.098679e+01
 * time: 378.7259681224823
    48     1.157868e+03     6.032481e+01
 * time: 385.16627192497253
    49     1.157864e+03     5.920991e+01
 * time: 392.3665211200714
    50     1.157856e+03     5.740417e+01
 * time: 399.12317299842834
    51     1.157832e+03     5.407031e+01
 * time: 405.5312600135803
    52     1.157774e+03     4.798188e+01
 * time: 411.8434569835663
    53     1.157628e+03     3.659183e+01
 * time: 417.7749660015106
    54     1.157299e+03     3.707862e+01
 * time: 423.7920289039612
    55     1.156690e+03     3.453183e+01
 * time: 430.7481029033661
    56     1.155959e+03     4.268473e+01
 * time: 437.5971009731293
    57     1.155557e+03     5.000309e+01
 * time: 444.66834592819214
    58     1.155472e+03     4.219411e+01
 * time: 451.15255403518677
    59     1.155463e+03     3.894982e+01
 * time: 457.10771799087524
    60     1.155462e+03     3.872028e+01
 * time: 463.0308039188385
    61     1.155458e+03     3.823435e+01
 * time: 468.9644401073456
    62     1.155450e+03     3.747328e+01
 * time: 475.63051199913025
    63     1.155426e+03     3.608641e+01
 * time: 484.16397309303284
    64     1.155365e+03     3.361168e+01
 * time: 490.96077609062195
    65     1.155207e+03     2.894844e+01
 * time: 497.47153401374817
    66     1.154814e+03     2.504332e+01
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    67     1.153910e+03     4.539953e+01
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 * time: 529.445424079895
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 * time: 535.3944590091705
    72     1.148932e+03     4.659470e+01
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    81     1.145347e+03     5.927932e+01
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    82     1.144536e+03     7.572710e+01
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    83     1.144354e+03     7.035223e+01
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    85     1.144340e+03     6.496308e+01
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    86     1.144333e+03     6.268335e+01
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    87     1.144320e+03     5.950296e+01
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    89     1.144186e+03     4.394749e+01
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    90     1.143949e+03     4.102392e+01
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    91     1.143423e+03     3.752367e+01
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    92     1.142498e+03     3.276758e+01
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    93     1.141490e+03     5.217551e+01
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    94     1.140969e+03     4.309381e+01
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    95     1.140872e+03     4.759588e+01
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    96     1.140867e+03     4.727133e+01
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    97     1.140866e+03     4.704459e+01
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    98     1.140862e+03     4.638036e+01
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    99     1.140853e+03     4.542717e+01
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   100     1.140829e+03     4.357571e+01
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   101     1.140767e+03     4.023368e+01
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   102     1.140611e+03     3.376885e+01
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   103     1.140237e+03     2.856928e+01
 * time: 741.5972790718079
   104     1.139452e+03     3.894107e+01
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   105     1.138224e+03     5.063688e+01
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   106     1.137140e+03     4.762519e+01
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   107     1.136763e+03     4.605051e+01
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   108     1.136720e+03     3.993288e+01
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   109     1.136717e+03     3.890566e+01
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   110     1.136716e+03     3.883647e+01
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   111     1.136712e+03     3.865523e+01
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   112     1.136702e+03     3.838845e+01
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   113     1.136674e+03     3.790005e+01
 * time: 802.9554579257965
   114     1.136604e+03     3.705604e+01
 * time: 809.3677079677582
   115     1.136418e+03     3.550032e+01
 * time: 815.57728099823
   116     1.135940e+03     3.257323e+01
 * time: 821.7186379432678
   117     1.134744e+03     2.697619e+01
 * time: 828.0183510780334
   118     1.132016e+03     4.861352e+01
 * time: 836.7732269763947
   119     1.127206e+03     6.900228e+01
 * time: 844.3954911231995
   120     1.122365e+03     5.795238e+01
 * time: 851.1173160076141
   121     1.119760e+03     2.908163e+01
 * time: 857.3920609951019
   122     1.118972e+03     2.137897e+01
 * time: 863.4135980606079
   123     1.118870e+03     1.909827e+01
 * time: 869.0985391139984
   124     1.118863e+03     1.886936e+01
 * time: 874.7888720035553
   125     1.118862e+03     1.852746e+01
 * time: 880.3354690074921
   126     1.118861e+03     1.864804e+01
 * time: 885.8741960525513
   127     1.118861e+03     1.862831e+01
 * time: 892.4295330047607
   128     1.118860e+03     1.865947e+01
 * time: 898.5750279426575
   129     1.118859e+03     1.869546e+01
 * time: 904.3966250419617
   130     1.118856e+03     1.878621e+01
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   131     1.118850e+03     1.894171e+01
 * time: 915.4731979370117
   132     1.118834e+03     1.922062e+01
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   133     1.118794e+03     1.967952e+01
 * time: 926.5905930995941
   134     1.118693e+03     2.042289e+01
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   135     1.118431e+03     2.157038e+01
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   136     1.117752e+03     2.595717e+01
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   137     1.116008e+03     4.715215e+01
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   138     1.111680e+03     7.743613e+01
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   139     1.102031e+03     7.499119e+01
 * time: 963.0916810035706
   140     1.090653e+03     6.250170e+01
 * time: 969.6220979690552
   141     1.087845e+03     8.960496e+01
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   142     1.087404e+03     1.190607e+02
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   143     1.085564e+03     6.087787e+01
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   144     1.085185e+03     1.947487e+01
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   145     1.085039e+03     1.959012e+01
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   146     1.084989e+03     1.990081e+01
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   147     1.084983e+03     2.009415e+01
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   148     1.084973e+03     2.065532e+01
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   149     1.084971e+03     2.079993e+01
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   150     1.084971e+03     2.084259e+01
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   151     1.084970e+03     2.084552e+01
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   152     1.084970e+03     2.084403e+01
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   153     1.084968e+03     2.084868e+01
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   154     1.084963e+03     2.088252e+01
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   155     1.084954e+03     2.098670e+01
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   156     1.084935e+03     2.121929e+01
 * time: 1086.025542974472
   157     1.084905e+03     2.160241e+01
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   158     1.084865e+03     2.207894e+01
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   159     1.084808e+03     2.259104e+01
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   160     1.084701e+03     2.318755e+01
 * time: 1113.2713129520416
   161     1.084458e+03     2.380805e+01
 * time: 1120.44814991951
   162     1.083881e+03     2.403001e+01
 * time: 1127.5383849143982
   163     1.082583e+03     2.865879e+01
 * time: 1134.552747964859
   164     1.081739e+03     3.341843e+01
 * time: 1142.3678998947144
   165     1.078499e+03     5.054188e+01
 * time: 1158.6822700500488
   166     1.075168e+03     6.966535e+01
 * time: 1166.51047706604
   167     1.069624e+03     6.533378e+01
 * time: 1174.2005801200867
   168     1.068098e+03     3.856272e+01
 * time: 1181.191729068756
   169     1.065571e+03     1.611062e+01
 * time: 1188.2503600120544
   170     1.065230e+03     1.527933e+01
 * time: 1196.1128549575806
   171     1.065088e+03     1.385503e+01
 * time: 1203.966567993164
   172     1.065086e+03     1.382742e+01
 * time: 1211.4150841236115
   173     1.065086e+03     1.383663e+01
 * time: 1218.9214730262756
   174     1.065086e+03     1.384273e+01
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   175     1.065086e+03     1.385109e+01
 * time: 1234.1082599163055
   176     1.065084e+03     1.386455e+01
 * time: 1246.8673739433289
   177     1.065082e+03     1.388591e+01
 * time: 1256.2646670341492
   178     1.065074e+03     1.391915e+01
 * time: 1264.3007459640503
   179     1.065053e+03     1.397116e+01
 * time: 1271.3638770580292
   180     1.065000e+03     1.742031e+01
 * time: 1278.5391728878021
   181     1.064862e+03     2.855619e+01
 * time: 1286.8038001060486
   182     1.064503e+03     4.585022e+01
 * time: 1294.2457818984985
   183     1.063581e+03     7.120893e+01
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   184     1.061297e+03     1.015565e+02
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   185     1.056901e+03     1.172079e+02
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   186     1.052919e+03     1.304962e+02
 * time: 1324.5694029331207
   187     1.048870e+03     1.445092e+02
 * time: 1332.6814370155334
   188     1.037225e+03     8.004893e+01
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   189     1.035989e+03     6.689301e+01
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   190     1.035708e+03     3.351073e+01
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   191     1.035220e+03     1.023182e+01
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   192     1.035148e+03     4.588384e+00
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   193     1.035107e+03     3.198226e+00
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   194     1.035086e+03     3.196151e+00
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   195     1.035067e+03     3.160387e+00
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   196     1.035054e+03     3.147012e+00
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   197     1.035047e+03     3.144947e+00
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   198     1.035046e+03     3.157031e+00
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   199     1.035046e+03     3.159475e+00
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   200     1.035046e+03     3.160059e+00
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   201     1.035046e+03     3.159779e+00
 * time: 1444.3073189258575
   202     1.035046e+03     3.158994e+00
 * time: 1451.2622609138489
   203     1.035046e+03     3.157781e+00
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   204     1.035046e+03     3.155534e+00
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   205     1.035045e+03     3.151677e+00
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   206     1.035044e+03     3.144628e+00
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   207     1.035041e+03     3.131349e+00
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   208     1.035033e+03     3.104858e+00
 * time: 1504.961014032364
   209     1.035012e+03     3.049096e+00
 * time: 1511.515860080719
   210     1.034957e+03     3.798101e+00
 * time: 1518.7099931240082
   211     1.034816e+03     6.029607e+00
 * time: 1525.9621739387512
   212     1.034472e+03     9.154140e+00
 * time: 1533.8677940368652
   213     1.033706e+03     1.258550e+01
 * time: 1542.0358819961548
   214     1.032296e+03     1.427962e+01
 * time: 1550.4064600467682
   215     1.030400e+03     1.353250e+01
 * time: 1559.6353030204773
   216     1.029562e+03     1.451097e+01
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   217     1.028793e+03     1.534582e+01
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   218     1.028722e+03     2.309928e+01
 * time: 1586.155445098877
   219     1.028409e+03     1.971948e+01
 * time: 1594.7229449748993
   220     1.028326e+03     1.936501e+01
 * time: 1604.319669008255
   221     1.028234e+03     2.040540e+01
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   222     1.028219e+03     2.201826e+01
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   223     1.028180e+03     2.177994e+01
 * time: 1629.965064048767
   224     1.028176e+03     2.186278e+01
 * time: 1638.4379448890686
   225     1.028175e+03     2.201733e+01
 * time: 1647.3765161037445
   226     1.028174e+03     2.197587e+01
 * time: 1654.9836270809174
   227     1.028174e+03     2.195893e+01
 * time: 1662.0927970409393
   228     1.028172e+03     2.186809e+01
 * time: 1669.8284730911255
   229     1.028169e+03     2.178132e+01
 * time: 1677.4585590362549
   230     1.028160e+03     2.162024e+01
 * time: 1684.5296549797058
   231     1.028139e+03     2.139041e+01
 * time: 1693.4997639656067
   232     1.028082e+03     2.100359e+01
 * time: 1701.5727980136871
   233     1.027936e+03     2.808708e+01
 * time: 1708.8303880691528
   234     1.027561e+03     4.544763e+01
 * time: 1715.8391830921173
   235     1.026628e+03     7.187426e+01
 * time: 1722.5530579090118
   236     1.024510e+03     1.065337e+02
 * time: 1729.3206329345703
   237     1.020621e+03     1.329824e+02
 * time: 1736.0531730651855
   238     1.015709e+03     1.045562e+02
 * time: 1742.3341069221497
   239     1.014365e+03     2.918128e+01
 * time: 1748.8578839302063
   240     1.013219e+03     1.803992e+01
 * time: 1757.0328419208527
   241     1.012886e+03     1.450533e+01
 * time: 1763.8864710330963
   242     1.012748e+03     9.039639e+00
 * time: 1770.166855096817
   243     1.012704e+03     2.255849e+00
 * time: 1776.3541219234467
   244     1.012698e+03     1.107778e+00
 * time: 1782.540020942688
   245     1.012697e+03     9.968755e-01
 * time: 1788.8844759464264
   246     1.012697e+03     9.970599e-01
 * time: 1794.840460062027
   247     1.012697e+03     9.971842e-01
 * time: 1801.0149629116058
   248     1.012697e+03     9.972169e-01
 * time: 1807.4633610248566
   249     1.012697e+03     9.972169e-01
 * time: 1814.7902998924255
   250     1.012697e+03     9.973243e-01
 * time: 1822.9457380771637
   251     1.012697e+03     9.973078e-01
 * time: 1830.299968957901
   252     1.012697e+03     9.972048e-01
 * time: 1837.031513929367
   253     1.012697e+03     9.970536e-01
 * time: 1843.599781036377
   254     1.012697e+03     9.967402e-01
 * time: 1850.5163431167603
   255     1.012697e+03     9.961565e-01
 * time: 1857.192148923874
   256     1.012696e+03     9.949478e-01
 * time: 1863.6648120880127
   257     1.012694e+03     1.312115e+00
 * time: 1870.3512001037598
   258     1.012690e+03     2.118030e+00
 * time: 1877.5871450901031
   259     1.012679e+03     3.352490e+00
 * time: 1884.6995720863342
   260     1.012651e+03     5.076526e+00
 * time: 1891.9568150043488
   261     1.012594e+03     6.891495e+00
 * time: 1898.9699161052704
   262     1.012501e+03     7.239074e+00
 * time: 1906.039745092392
   263     1.012418e+03     4.479611e+00
 * time: 1913.6677079200745
   264     1.012390e+03     1.267149e+00
 * time: 1921.5598249435425
   265     1.012386e+03     8.840304e-01
 * time: 1929.2193241119385
   266     1.012386e+03     8.549097e-01
 * time: 1936.2232670783997
   267     1.012386e+03     8.516869e-01
 * time: 1942.9492959976196
   268     1.012386e+03     8.516869e-01
 * time: 1950.5795691013336
   269     1.012386e+03     8.516869e-01
 * time: 1958.534110069275
   270     1.012386e+03     8.516817e-01
 * time: 1965.7023429870605
   271     1.012386e+03     8.516817e-01
 * time: 1974.8013410568237
   272     1.012386e+03     8.516817e-01
 * time: 1983.5860090255737
   273     1.012386e+03     8.516817e-01
 * time: 1991.9630680084229
   274     1.012386e+03     8.516817e-01
 * time: 2000.6564168930054
   275     1.012386e+03     8.516817e-01
 * time: 2009.2055430412292
FittedPumasModel

Successful minimization:                      true

Likelihood approximation:                     FOCE
Likelihood Optimizer:                         BFGS
Dynamical system type:               Nonlinear ODE
Solver(s):                  OrdinaryDiffEq.Rodas5P

Log-likelihood value:                   -1012.3861
Number of subjects:                             31
Number of parameters:         Fixed      Optimized
                                  0             18
Observation records:         Active        Missing
    conc:                       239             47
    pca:                        225             61
    Total:                      464            108

--------------------------
              Estimate
--------------------------
pop_CL         0.13136
pop_V          8.0217
pop_tabs       0.41997
pop_lag        0.92474
pop_e0        96.692
pop_emax      -1.0629
pop_c50        1.5003
pop_tover     14.06
pk_Ω₁,₁        0.05162
pk_Ω₂,₂        0.019945
pk_Ω₃,₃        1.0369
pd_Ω₁,₁        0.0025468
pd_Ω₂,₂        0.00037903
pd_Ω₃,₃        0.15787
pd_Ω₄,₄        0.012052
σ_prop         0.088654
σ_add          0.36167
σ_fx           3.5399
--------------------------

On the other hand, if it is known that a differential equation is non-stiff (this might be difficult to guarantee for all admissible parameter values), a non-stiff solver such as Tsit5 at high tolerances or Vern7 at low tolerances could be an alternative to the default solver:

# Fitting with the non-stiff solver Vern7 at low tolerances (relative: 1e-8, absolute: 1e-12)
fit(
    warfarin_pkpd_model,
    pop,
    init_params(warfarin_pkpd_model),
    FOCE();
    diffeq_options = (; alg = Vern7(), reltol = 1e-8, abstol = 1e-12),
)
[ Info: Checking the initial parameter values.
[ Info: The initial negative log likelihood and its gradient are finite. Check passed.
Iter     Function value   Gradient norm 
     0     2.998927e+06     5.740466e+06
 * time: 7.700920104980469e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.377593994140625
     2     3.595691e+05     6.035843e+05
 * time: 2.6294469833374023
     3     1.691071e+05     2.702705e+05
 * time: 3.7249698638916016
     4     9.193152e+04     1.451040e+05
 * time: 4.860307931900024
     5     4.768539e+04     6.630202e+04
 * time: 5.905808925628662
     6     2.904205e+04     3.416919e+04
 * time: 6.976714849472046
     7     1.840153e+04     1.671742e+04
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   247     1.012697e+03     9.972237e-01
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   248     1.012697e+03     9.972927e-01
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   249     1.012697e+03     9.973804e-01
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   250     1.012697e+03     9.975286e-01
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   251     1.012697e+03     9.977461e-01
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   252     1.012697e+03     9.980635e-01
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   253     1.012697e+03     9.984743e-01
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   255     1.012694e+03     9.988363e-01
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   256     1.012690e+03     1.474525e+00
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   257     1.012678e+03     2.326520e+00
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   258     1.012649e+03     3.510529e+00
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   259     1.012590e+03     4.737310e+00
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   260     1.012497e+03     4.897808e+00
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   261     1.012416e+03     2.875911e+00
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   262     1.012389e+03     9.301211e-01
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   265     1.012386e+03     8.516621e-01
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   266     1.012386e+03     8.511780e-01
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   267     1.012386e+03     8.489977e-01
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   268     1.012386e+03     8.463566e-01
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   269     1.012386e+03     8.415231e-01
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   270     1.012386e+03     8.340037e-01
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   271     1.012386e+03     8.215489e-01
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   272     1.012386e+03     8.013257e-01
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   273     1.012385e+03     7.680260e-01
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   274     1.012383e+03     7.407512e-01
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   275     1.012377e+03     7.469071e-01
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   276     1.012364e+03     1.103988e+00
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   277     1.012330e+03     1.710981e+00
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   278     1.012257e+03     2.412723e+00
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   279     1.012125e+03     2.705165e+00
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   280     1.011948e+03     1.883597e+00
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   281     1.011809e+03     8.215973e-01
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   282     1.011775e+03     9.067203e-01
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   283     1.011774e+03     9.565073e-01
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   284     1.011774e+03     9.628263e-01
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   285     1.011774e+03     9.654767e-01
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   286     1.011774e+03     9.654769e-01
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   287     1.011774e+03     9.656366e-01
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   288     1.011774e+03     9.656622e-01
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   294     1.011774e+03     9.662851e-01
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   295     1.011773e+03     9.654773e-01
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   296     1.011772e+03     9.623124e-01
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   299     1.011746e+03     9.242954e-01
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   300     1.011711e+03     1.257052e+00
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   301     1.011651e+03     1.361278e+00
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   302     1.011591e+03     9.076916e-01
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   303     1.011568e+03     3.084752e-01
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   304     1.011565e+03     2.946404e-01
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   305     1.011565e+03     2.821273e-01
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   306     1.011565e+03     2.766260e-01
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   307     1.011565e+03     2.778010e-01
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   308     1.011565e+03     2.830281e-01
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   309     1.011565e+03     2.842619e-01
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   310     1.011565e+03     2.861093e-01
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   311     1.011565e+03     2.861138e-01
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   312     1.011565e+03     2.867641e-01
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   313     1.011565e+03     2.867756e-01
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   314     1.011565e+03     2.884589e-01
 * time: 346.20126700401306
   315     1.011565e+03     2.897335e-01
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   316     1.011565e+03     2.927744e-01
 * time: 348.597354888916
   317     1.011565e+03     2.967424e-01
 * time: 349.8204870223999
   318     1.011565e+03     3.027347e-01
 * time: 351.0485370159149
   319     1.011564e+03     3.099827e-01
 * time: 352.32139587402344
   320     1.011563e+03     3.155826e-01
 * time: 353.5335659980774
   321     1.011560e+03     3.082844e-01
 * time: 354.77706694602966
   322     1.011555e+03     2.625121e-01
 * time: 356.0280499458313
   323     1.011548e+03     2.136271e-01
 * time: 357.24846482276917
   324     1.011542e+03     1.165033e-01
 * time: 358.4999248981476
   325     1.011538e+03     3.914224e-02
 * time: 359.73978090286255
   326     1.011537e+03     3.083888e-02
 * time: 361.0262989997864
   327     1.011536e+03     2.766364e-02
 * time: 362.2369840145111
   328     1.011536e+03     1.873262e-02
 * time: 363.535502910614
   329     1.011536e+03     5.453951e-03
 * time: 364.74417090415955
   330     1.011536e+03     4.882951e-03
 * time: 366.0375728607178
   331     1.011535e+03     4.331879e-03
 * time: 367.34556889533997
   332     1.011535e+03     2.711268e-03
 * time: 368.60181498527527
   333     1.011535e+03     4.474256e-04
 * time: 369.93529295921326
FittedPumasModel

Successful minimization:                      true

Likelihood approximation:                     FOCE
Likelihood Optimizer:                         BFGS
Dynamical system type:               Nonlinear ODE
Solver(s):                    OrdinaryDiffEq.Vern7

Log-likelihood value:                   -1011.5354
Number of subjects:                             31
Number of parameters:         Fixed      Optimized
                                  0             18
Observation records:         Active        Missing
    conc:                       239             47
    pca:                        225             61
    Total:                      464            108

-------------------------
              Estimate
-------------------------
pop_CL         0.13137
pop_V          8.0255
pop_tabs       0.42635
pop_lag        0.92336
pop_e0        96.688
pop_emax      -1.0628
pop_c50        1.5016
pop_tover     14.055
pk_Ω₁,₁        0.050744
pk_Ω₂,₂        0.021666
pk_Ω₃,₃        0.97884
pd_Ω₁,₁        0.0028828
pd_Ω₂,₂        7.3726e-9
pd_Ω₃,₃        0.15184
pd_Ω₄,₄        0.011913
σ_prop         0.088292
σ_add          0.36362
σ_fx           3.5336
-------------------------

8 Concluding Remarks

In this tutorial, you have seen how to adjust the tolerances and the algorithm of the differential solver. Usually, the default differential equation solver in Pumas is an efficient choice. To reduce numerical issues, sometimes it can be helpful to decrease the default tolerances.