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: 0.0339818000793457
     1     4.816669e+05     8.316173e+05
 * time: 1.7958779335021973
     2     3.598373e+05     6.060150e+05
 * time: 2.507612943649292
     3     1.682124e+05     2.709072e+05
 * time: 3.1302638053894043
     4     9.168914e+04     1.446832e+05
 * time: 3.8037188053131104
     5     4.791138e+04     6.671463e+04
 * time: 4.472915887832642
     6     2.909104e+04     3.426663e+04
 * time: 5.085271835327148
     7     1.843737e+04     1.678953e+04
 * time: 5.820868968963623
     8     1.282447e+04     1.021463e+04
 * time: 6.456052780151367
     9     9.723361e+03     9.291650e+03
 * time: 7.130998849868774
    10     7.635613e+03     8.325311e+03
 * time: 7.7556328773498535
    11     6.243129e+03     7.337397e+03
 * time: 8.4772629737854
    12     5.279957e+03     6.304542e+03
 * time: 9.089170932769775
    13     4.422123e+03     5.001962e+03
 * time: 9.71747088432312
    14     3.744748e+03     3.550400e+03
 * time: 10.330431938171387
    15     3.409674e+03     2.461547e+03
 * time: 10.950011968612671
    16     3.312049e+03     1.856430e+03
 * time: 11.597062826156616
    17     3.300559e+03     1.648439e+03
 * time: 12.17012882232666
    18     3.299489e+03     1.599460e+03
 * time: 12.790802001953125
    19     3.298436e+03     1.567520e+03
 * time: 13.358941793441772
    20     3.294921e+03     1.501363e+03
 * time: 13.995409965515137
    21     3.286641e+03     1.407664e+03
 * time: 14.590467929840088
    22     3.264637e+03     1.259077e+03
 * time: 15.220343828201294
    23     3.209594e+03     1.043534e+03
 * time: 15.79503083229065
    24     3.072117e+03     7.454677e+02
 * time: 16.42513084411621
    25     2.739794e+03     3.818063e+02
 * time: 16.97980284690857
    26     1.949883e+03     2.246644e+02
 * time: 17.57834792137146
    27     1.722434e+03     2.224562e+02
 * time: 19.189254999160767
    28     1.479355e+03     2.042452e+02
 * time: 21.31681489944458
    29     1.310583e+03     1.531868e+02
 * time: 23.79111886024475
    30     1.224675e+03     1.756826e+02
 * time: 24.416525840759277
    31     1.204841e+03     2.738440e+02
 * time: 24.94250798225403
    32     1.196432e+03     2.403217e+02
 * time: 25.503607988357544
    33     1.189131e+03     2.301186e+02
 * time: 26.02058696746826
    34     1.177881e+03     2.083030e+02
 * time: 26.570385932922363
    35     1.176989e+03     2.201993e+02
 * time: 27.106979846954346
    36     1.176899e+03     2.230826e+02
 * time: 27.701766967773438
    37     1.176882e+03     2.233531e+02
 * time: 28.23801898956299
    38     1.176795e+03     2.238567e+02
 * time: 28.7918119430542
    39     1.176609e+03     2.236914e+02
 * time: 29.3258318901062
    40     1.176090e+03     2.210941e+02
 * time: 29.870957851409912
    41     1.174841e+03     2.112741e+02
 * time: 30.37274694442749
    42     1.171973e+03     1.825298e+02
 * time: 30.902796983718872
    43     1.166737e+03     1.210559e+02
 * time: 31.444140911102295
    44     1.160698e+03     4.450697e+01
 * time: 31.998480796813965
    45     1.157926e+03     6.411275e+01
 * time: 32.57678198814392
    46     1.157537e+03     6.121762e+01
 * time: 33.10056495666504
    47     1.157520e+03     5.867223e+01
 * time: 33.621803998947144
    48     1.157519e+03     5.814523e+01
 * time: 34.09947180747986
    49     1.157515e+03     5.686007e+01
 * time: 34.64080595970154
    50     1.157505e+03     5.497418e+01
 * time: 35.138354778289795
    51     1.157477e+03     5.129724e+01
 * time: 35.726433992385864
    52     1.157408e+03     4.463171e+01
 * time: 36.210612773895264
    53     1.157239e+03     3.279117e+01
 * time: 36.76032495498657
    54     1.156869e+03     3.681526e+01
 * time: 37.2630558013916
    55     1.156230e+03     3.198754e+01
 * time: 37.82084584236145
    56     1.155563e+03     4.576438e+01
 * time: 38.33037495613098
    57     1.155264e+03     4.832159e+01
 * time: 38.89198684692383
    58     1.155212e+03     4.052175e+01
 * time: 39.3912079334259
    59     1.155207e+03     3.797125e+01
 * time: 39.91742181777954
    60     1.155206e+03     3.778452e+01
 * time: 40.38673996925354
    61     1.155201e+03     3.723003e+01
 * time: 40.910271883010864
    62     1.155190e+03     3.642375e+01
 * time: 41.397350788116455
    63     1.155159e+03     3.487535e+01
 * time: 41.91043186187744
    64     1.155079e+03     3.209060e+01
 * time: 42.38745880126953
    65     1.154875e+03     2.670227e+01
 * time: 42.90211486816406
    66     1.154375e+03     2.366165e+01
 * time: 43.48126697540283
    67     1.153278e+03     5.499525e+01
 * time: 44.012794971466064
    68     1.151457e+03     7.903833e+01
 * time: 44.506664991378784
    69     1.149737e+03     6.771227e+01
 * time: 45.01737189292908
    70     1.148983e+03     5.387708e+01
 * time: 45.48857498168945
    71     1.148872e+03     4.683950e+01
 * time: 45.988075971603394
    72     1.148868e+03     4.512036e+01
 * time: 46.451666831970215
    73     1.148866e+03     4.489706e+01
 * time: 46.940133810043335
    74     1.148857e+03     4.426314e+01
 * time: 47.46504783630371
    75     1.148838e+03     4.321161e+01
 * time: 47.96266984939575
    76     1.148785e+03     4.095232e+01
 * time: 48.45963191986084
    77     1.148652e+03     3.780572e+01
 * time: 48.975937843322754
    78     1.148325e+03     3.474239e+01
 * time: 49.467108964920044
    79     1.147599e+03     3.678957e+01
 * time: 49.98815894126892
    80     1.146330e+03     5.134277e+01
 * time: 50.480265855789185
    81     1.144979e+03     6.444063e+01
 * time: 51.01074695587158
    82     1.144325e+03     7.574751e+01
 * time: 51.511841773986816
    83     1.144210e+03     6.968296e+01
 * time: 52.025261878967285
    84     1.144204e+03     6.648234e+01
 * time: 52.524866819381714
    85     1.144202e+03     6.560999e+01
 * time: 53.03499698638916
    86     1.144194e+03     6.305667e+01
 * time: 53.522565841674805
    87     1.144176e+03     5.956959e+01
 * time: 54.02125096321106
    88     1.144127e+03     5.307828e+01
 * time: 54.512510776519775
    89     1.144005e+03     4.218776e+01
 * time: 55.00678992271423
    90     1.143708e+03     4.120261e+01
 * time: 55.544227838516235
    91     1.143074e+03     3.675516e+01
 * time: 56.00844192504883
    92     1.142052e+03     3.855439e+01
 * time: 56.54326295852661
    93     1.141101e+03     5.193197e+01
 * time: 57.021976947784424
    94     1.140701e+03     4.606858e+01
 * time: 57.55663800239563
    95     1.140646e+03     4.857736e+01
 * time: 58.076167821884155
    96     1.140643e+03     4.817596e+01
 * time: 58.64153981208801
    97     1.140642e+03     4.793197e+01
 * time: 59.11133694648743
    98     1.140637e+03     4.723245e+01
 * time: 59.64469599723816
    99     1.140626e+03     4.619695e+01
 * time: 60.12525200843811
   100     1.140597e+03     4.418761e+01
 * time: 60.66202783584595
   101     1.140523e+03     4.050986e+01
 * time: 61.164766788482666
   102     1.140334e+03     3.335334e+01
 * time: 61.73862981796265
   103     1.139892e+03     2.982120e+01
 * time: 62.23499083518982
   104     1.138994e+03     4.360237e+01
 * time: 62.77519989013672
   105     1.137687e+03     5.246892e+01
 * time: 63.30396795272827
   106     1.136679e+03     4.825586e+01
 * time: 63.896960973739624
   107     1.136391e+03     4.467499e+01
 * time: 64.4011697769165
   108     1.136364e+03     3.961591e+01
 * time: 64.94612097740173
   109     1.136362e+03     3.951402e+01
 * time: 65.45155787467957
   110     1.136360e+03     3.943358e+01
 * time: 65.9938600063324
   111     1.136355e+03     3.923965e+01
 * time: 66.49695897102356
   112     1.136344e+03     3.894674e+01
 * time: 67.04910278320312
   113     1.136314e+03     3.841518e+01
 * time: 67.51975679397583
   114     1.136235e+03     3.748824e+01
 * time: 68.10331797599792
   115     1.136029e+03     3.577210e+01
 * time: 68.59290981292725
   116     1.135499e+03     3.252332e+01
 * time: 69.10163378715515
   117     1.134182e+03     2.630844e+01
 * time: 69.5749499797821
   118     1.131238e+03     4.869163e+01
 * time: 70.07994985580444
   119     1.126345e+03     6.537856e+01
 * time: 70.55313086509705
   120     1.121879e+03     5.123373e+01
 * time: 71.04973292350769
   121     1.119645e+03     2.457316e+01
 * time: 71.53385186195374
   122     1.119031e+03     2.124721e+01
 * time: 72.0518548488617
   123     1.118958e+03     1.857508e+01
 * time: 72.56521677970886
   124     1.118953e+03     1.896595e+01
 * time: 73.07893490791321
   125     1.118952e+03     1.849021e+01
 * time: 73.56636095046997
   126     1.118952e+03     1.852941e+01
 * time: 74.05266284942627
   127     1.118951e+03     1.856812e+01
 * time: 74.52189493179321
   128     1.118951e+03     1.862403e+01
 * time: 75.00643491744995
   129     1.118949e+03     1.871484e+01
 * time: 75.4756247997284
   130     1.118946e+03     1.886332e+01
 * time: 75.9723379611969
   131     1.118939e+03     1.910217e+01
 * time: 76.44840693473816
   132     1.118922e+03     1.948050e+01
 * time: 76.9343090057373
   133     1.118879e+03     2.006903e+01
 * time: 77.41065096855164
   134     1.118769e+03     2.097106e+01
 * time: 77.93490481376648
   135     1.118486e+03     2.231801e+01
 * time: 78.47754192352295
   136     1.117755e+03     3.049039e+01
 * time: 78.94086384773254
   137     1.115883e+03     5.345487e+01
 * time: 79.46524977684021
   138     1.111258e+03     8.489274e+01
 * time: 79.93238282203674
   139     1.101149e+03     7.643749e+01
 * time: 80.45492577552795
   140     1.090612e+03     5.802659e+01
 * time: 80.96899485588074
   141     1.087971e+03     5.776554e+01
 * time: 81.53250288963318
   142     1.087205e+03     1.148234e+02
 * time: 82.08293795585632
   143     1.085558e+03     4.724872e+01
 * time: 82.67391180992126
   144     1.085271e+03     2.018848e+01
 * time: 83.2771577835083
   145     1.085124e+03     2.004201e+01
 * time: 83.8490629196167
   146     1.085089e+03     2.025616e+01
 * time: 84.40231895446777
   147     1.085081e+03     2.049682e+01
 * time: 84.9927728176117
   148     1.085073e+03     2.086075e+01
 * time: 85.59928178787231
   149     1.085070e+03     2.095706e+01
 * time: 86.14397096633911
   150     1.085063e+03     2.094978e+01
 * time: 86.73586297035217
   151     1.085062e+03     2.093855e+01
 * time: 87.32943677902222
   152     1.085062e+03     2.093439e+01
 * time: 88.04063081741333
   153     1.085062e+03     2.093335e+01
 * time: 88.75857186317444
   154     1.085062e+03     2.093252e+01
 * time: 89.52643394470215
   155     1.085062e+03     2.093227e+01
 * time: 90.3319799900055
   156     1.085062e+03     2.093208e+01
 * time: 91.15072584152222
   157     1.085062e+03     2.093187e+01
 * time: 91.90822696685791
   158     1.085062e+03     2.093181e+01
 * time: 92.74485397338867
   159     1.085062e+03     2.093175e+01
 * time: 93.61421489715576
   160     1.085062e+03     2.093172e+01
 * time: 94.50738286972046
   161     1.085062e+03     2.046653e+01
 * time: 95.04513788223267
   162     1.085061e+03     2.045920e+01
 * time: 95.69139289855957
   163     1.085056e+03     2.021786e+01
 * time: 96.24114179611206
   164     1.085055e+03     2.018381e+01
 * time: 96.87740087509155
   165     1.085043e+03     1.997498e+01
 * time: 97.42394089698792
   166     1.085029e+03     1.961695e+01
 * time: 98.00298190116882
   167     1.084997e+03     1.929634e+01
 * time: 98.58177089691162
   168     1.084881e+03     1.855033e+01
 * time: 99.16992378234863
   169     1.084633e+03     1.799616e+01
 * time: 99.81237387657166
   170     1.083831e+03     1.856893e+01
 * time: 100.33554291725159
   171     1.081240e+03     2.069096e+01
 * time: 100.92425990104675
   172     1.072465e+03     3.262436e+01
 * time: 101.53254890441895
   173     1.072446e+03     6.686587e+01
 * time: 102.17868280410767
   174     1.068493e+03     7.417042e+01
 * time: 102.81279182434082
   175     1.067469e+03     7.109936e+01
 * time: 103.55252981185913
   176     1.065854e+03     2.774005e+01
 * time: 104.19124388694763
   177     1.065215e+03     1.328875e+01
 * time: 104.8390998840332
   178     1.065128e+03     1.390400e+01
 * time: 105.47695088386536
   179     1.065120e+03     1.383172e+01
 * time: 106.13209700584412
   180     1.065120e+03     1.384995e+01
 * time: 106.79382586479187
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: 6.604194641113281e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.359144926071167
     2     3.595691e+05     6.035843e+05
 * time: 2.5115089416503906
     3     1.691071e+05     2.702705e+05
 * time: 3.7016608715057373
     4     9.193152e+04     1.451040e+05
 * time: 4.9254279136657715
     5     4.768539e+04     6.630202e+04
 * time: 6.0788819789886475
     6     2.904205e+04     3.416919e+04
 * time: 7.245415925979614
     7     1.840153e+04     1.671742e+04
 * time: 8.365715980529785
     8     1.280989e+04     1.022268e+04
 * time: 9.426537036895752
     9     9.713287e+03     9.298485e+03
 * time: 10.463114023208618
    10     7.628127e+03     8.330656e+03
 * time: 11.513552904129028
    11     6.236278e+03     7.340367e+03
 * time: 12.598955869674683
    12     5.271958e+03     6.303773e+03
 * time: 13.608495950698853
    13     4.411047e+03     4.994130e+03
 * time: 14.662185907363892
    14     3.732936e+03     3.538184e+03
 * time: 15.62900686264038
    15     3.399273e+03     2.450045e+03
 * time: 16.63200283050537
    16     3.302987e+03     1.848476e+03
 * time: 17.62273097038269
    17     3.291788e+03     1.642864e+03
 * time: 18.689509868621826
    18     3.290743e+03     1.594765e+03
 * time: 19.763722896575928
    19     3.289687e+03     1.562641e+03
 * time: 20.8901789188385
    20     3.286186e+03     1.496632e+03
 * time: 21.987637996673584
    21     3.277917e+03     1.402865e+03
 * time: 23.094738006591797
    22     3.255959e+03     1.254297e+03
 * time: 24.18255591392517
    23     3.200980e+03     1.038621e+03
 * time: 25.291775941848755
    24     3.063521e+03     7.403605e+02
 * time: 26.318245887756348
    25     2.730316e+03     3.768701e+02
 * time: 27.356482982635498
    26     1.934186e+03     2.246121e+02
 * time: 28.32728886604309
    27     1.704399e+03     2.219826e+02
 * time: 32.265588998794556
    28     1.471183e+03     2.025826e+02
 * time: 39.179713010787964
    29     1.316789e+03     1.554714e+02
 * time: 45.10893201828003
    30     1.226363e+03     1.775666e+02
 * time: 46.24491500854492
    31     1.206626e+03     2.780980e+02
 * time: 47.27215600013733
    32     1.197830e+03     2.430550e+02
 * time: 48.32266402244568
    33     1.190342e+03     2.327905e+02
 * time: 49.34406399726868
    34     1.178783e+03     2.105809e+02
 * time: 50.40811896324158
    35     1.177853e+03     2.228585e+02
 * time: 51.44653797149658
    36     1.177763e+03     2.256116e+02
 * time: 52.495179891586304
    37     1.177745e+03     2.259359e+02
 * time: 53.55114197731018
    38     1.177670e+03     2.264633e+02
 * time: 54.55349588394165
    39     1.177500e+03     2.264574e+02
 * time: 55.615402936935425
    40     1.177033e+03     2.244136e+02
 * time: 56.611177921295166
    41     1.175894e+03     2.160226e+02
 * time: 57.680712938308716
    42     1.173240e+03     1.905912e+02
 * time: 58.690356969833374
    43     1.168189e+03     1.332783e+02
 * time: 59.926997900009155
    44     1.161835e+03     5.289952e+01
 * time: 60.978893995285034
    45     1.158453e+03     6.595387e+01
 * time: 62.0408878326416
    46     1.157897e+03     6.411419e+01
 * time: 63.76893901824951
    47     1.157870e+03     6.098509e+01
 * time: 65.14499688148499
    48     1.157868e+03     6.032325e+01
 * time: 66.26513695716858
    49     1.157864e+03     5.920812e+01
 * time: 67.37696981430054
    50     1.157856e+03     5.740225e+01
 * time: 68.41786289215088
    51     1.157832e+03     5.406795e+01
 * time: 69.53313183784485
    52     1.157774e+03     4.797879e+01
 * time: 70.50396585464478
    53     1.157628e+03     3.658732e+01
 * time: 71.56993889808655
    54     1.157299e+03     3.707874e+01
 * time: 72.57355999946594
    55     1.156690e+03     3.452947e+01
 * time: 73.63233995437622
    56     1.155959e+03     4.269004e+01
 * time: 74.63366985321045
    57     1.155557e+03     5.000322e+01
 * time: 75.67968583106995
    58     1.155472e+03     4.219349e+01
 * time: 76.77702593803406
    59     1.155463e+03     3.895049e+01
 * time: 77.95550990104675
    60     1.155462e+03     3.872099e+01
 * time: 78.95889782905579
    61     1.155458e+03     3.823491e+01
 * time: 79.97477293014526
    62     1.155450e+03     3.747370e+01
 * time: 80.94057583808899
    63     1.155426e+03     3.608648e+01
 * time: 81.9850549697876
    64     1.155365e+03     3.361118e+01
 * time: 82.94562292098999
    65     1.155207e+03     2.894678e+01
 * time: 83.93896293640137
    66     1.154814e+03     2.504284e+01
 * time: 84.88165998458862
    67     1.153910e+03     4.541126e+01
 * time: 85.88738799095154
    68     1.152230e+03     7.426044e+01
 * time: 86.83668303489685
    69     1.150286e+03     7.473044e+01
 * time: 87.84930992126465
    70     1.149188e+03     5.605967e+01
 * time: 88.86928486824036
    71     1.148944e+03     4.801123e+01
 * time: 89.9883189201355
    72     1.148931e+03     4.659478e+01
 * time: 90.99528384208679
    73     1.148929e+03     4.635180e+01
 * time: 92.02197980880737
    74     1.148922e+03     4.568579e+01
 * time: 93.01829481124878
    75     1.148907e+03     4.466128e+01
 * time: 94.07059288024902
    76     1.148864e+03     4.249742e+01
 * time: 95.35555386543274
    77     1.148758e+03     3.824840e+01
 * time: 96.83344197273254
    78     1.148490e+03     3.552459e+01
 * time: 97.95430898666382
    79     1.147881e+03     3.038181e+01
 * time: 99.09204983711243
    80     1.146736e+03     4.467281e+01
 * time: 100.12004494667053
    81     1.145347e+03     5.929158e+01
 * time: 101.21826100349426
    82     1.144535e+03     7.572784e+01
 * time: 102.25839686393738
    83     1.144354e+03     7.034986e+01
 * time: 103.32884383201599
    84     1.144341e+03     6.591584e+01
 * time: 104.32950401306152
    85     1.144340e+03     6.496353e+01
 * time: 105.36305785179138
    86     1.144333e+03     6.268267e+01
 * time: 106.34987497329712
    87     1.144320e+03     5.950121e+01
 * time: 107.35847902297974
    88     1.144281e+03     5.368613e+01
 * time: 108.43915891647339
    89     1.144186e+03     4.393956e+01
 * time: 109.41955399513245
    90     1.143949e+03     4.102324e+01
 * time: 110.55263185501099
    91     1.143423e+03     3.752005e+01
 * time: 111.69638586044312
    92     1.142497e+03     3.278884e+01
 * time: 112.97472786903381
    93     1.141489e+03     5.217814e+01
 * time: 114.0677649974823
    94     1.140969e+03     4.310032e+01
 * time: 115.12609100341797
    95     1.140872e+03     4.759545e+01
 * time: 116.12950992584229
    96     1.140867e+03     4.727034e+01
 * time: 117.14846682548523
    97     1.140866e+03     4.704365e+01
 * time: 118.15179085731506
    98     1.140862e+03     4.637923e+01
 * time: 119.37341403961182
    99     1.140853e+03     4.542582e+01
 * time: 120.37834596633911
   100     1.140829e+03     4.357378e+01
 * time: 121.50409197807312
   101     1.140767e+03     4.023060e+01
 * time: 122.49957299232483
   102     1.140610e+03     3.376323e+01
 * time: 123.55615496635437
   103     1.140237e+03     2.857028e+01
 * time: 124.62670993804932
   104     1.139451e+03     3.895699e+01
 * time: 125.68165802955627
   105     1.138223e+03     5.064036e+01
 * time: 126.68359804153442
   106     1.137140e+03     4.763314e+01
 * time: 128.171856880188
   107     1.136763e+03     4.604968e+01
 * time: 129.45040488243103
   108     1.136720e+03     3.993479e+01
 * time: 130.5683560371399
   109     1.136717e+03     3.890565e+01
 * time: 131.54414200782776
   110     1.136716e+03     3.883645e+01
 * time: 132.56551694869995
   111     1.136712e+03     3.865517e+01
 * time: 133.5807809829712
   112     1.136702e+03     3.838833e+01
 * time: 134.61298894882202
   113     1.136674e+03     3.789981e+01
 * time: 135.590313911438
   114     1.136603e+03     3.705558e+01
 * time: 136.60392904281616
   115     1.136418e+03     3.549940e+01
 * time: 137.58259892463684
   116     1.135940e+03     3.257136e+01
 * time: 138.62982892990112
   117     1.134743e+03     2.697248e+01
 * time: 139.6189320087433
   118     1.132014e+03     4.863416e+01
 * time: 140.63653993606567
   119     1.127203e+03     6.901202e+01
 * time: 141.6482539176941
   120     1.122363e+03     5.794311e+01
 * time: 142.83735489845276
   121     1.119759e+03     2.906766e+01
 * time: 143.86038994789124
   122     1.118972e+03     2.137843e+01
 * time: 144.90088486671448
   123     1.118870e+03     1.909639e+01
 * time: 145.9658648967743
   124     1.118863e+03     1.887024e+01
 * time: 147.06253790855408
   125     1.118862e+03     1.852691e+01
 * time: 148.1855800151825
   126     1.118861e+03     1.864783e+01
 * time: 149.21927690505981
   127     1.118861e+03     1.862870e+01
 * time: 150.32411289215088
   128     1.118860e+03     1.866000e+01
 * time: 151.50483798980713
   129     1.118859e+03     1.869625e+01
 * time: 152.5109989643097
   130     1.118856e+03     1.878717e+01
 * time: 153.53188586235046
   131     1.118850e+03     1.894283e+01
 * time: 154.5810239315033
   132     1.118834e+03     1.922180e+01
 * time: 155.61012291908264
   133     1.118794e+03     1.968060e+01
 * time: 156.6268048286438
   134     1.118693e+03     2.042353e+01
 * time: 157.71284985542297
   135     1.118431e+03     2.156998e+01
 * time: 158.7893009185791
   136     1.117751e+03     2.596600e+01
 * time: 159.8843059539795
   137     1.116007e+03     4.716393e+01
 * time: 161.01157188415527
   138     1.111677e+03     7.744992e+01
 * time: 162.0626859664917
   139     1.102024e+03     7.496268e+01
 * time: 163.16112780570984
   140     1.090649e+03     6.246756e+01
 * time: 164.20898485183716
   141     1.087843e+03     8.941834e+01
 * time: 165.35396695137024
   142     1.087400e+03     1.188539e+02
 * time: 166.4546799659729
   143     1.085563e+03     6.082023e+01
 * time: 167.6016058921814
   144     1.085185e+03     1.947062e+01
 * time: 168.76645803451538
   145     1.085039e+03     1.958695e+01
 * time: 169.90019488334656
   146     1.084989e+03     1.989918e+01
 * time: 171.02672386169434
   147     1.084983e+03     2.009395e+01
 * time: 172.18368697166443
   148     1.084973e+03     2.065564e+01
 * time: 173.3060028553009
   149     1.084971e+03     2.080010e+01
 * time: 174.44193696975708
   150     1.084971e+03     2.084232e+01
 * time: 175.54796385765076
   151     1.084970e+03     2.084506e+01
 * time: 176.6801040172577
   152     1.084970e+03     2.084321e+01
 * time: 177.79180598258972
   153     1.084968e+03     2.084737e+01
 * time: 178.92061281204224
   154     1.084963e+03     2.088045e+01
 * time: 180.0184519290924
   155     1.084954e+03     2.098376e+01
 * time: 181.15070390701294
   156     1.084935e+03     2.121619e+01
 * time: 182.2330038547516
   157     1.084904e+03     2.160158e+01
 * time: 183.29005789756775
   158     1.084864e+03     2.208328e+01
 * time: 184.37967586517334
   159     1.084807e+03     2.260004e+01
 * time: 185.42147183418274
   160     1.084700e+03     2.319870e+01
 * time: 186.5888798236847
   161     1.084457e+03     2.382124e+01
 * time: 187.68753290176392
   162     1.083882e+03     2.406095e+01
 * time: 188.87911081314087
   163     1.082589e+03     2.849219e+01
 * time: 189.9885368347168
   164     1.081660e+03     3.379635e+01
 * time: 191.36220502853394
   165     1.078576e+03     4.983497e+01
 * time: 192.75205302238464
   166     1.075004e+03     6.870527e+01
 * time: 194.07909393310547
   167     1.069377e+03     6.309176e+01
 * time: 195.30575394630432
   168     1.067978e+03     3.811944e+01
 * time: 196.5279769897461
   169     1.065604e+03     1.825143e+01
 * time: 197.73960185050964
   170     1.065251e+03     1.739631e+01
 * time: 198.9821138381958
   171     1.065088e+03     1.384781e+01
 * time: 200.2135169506073
   172     1.065086e+03     1.382386e+01
 * time: 201.3877468109131
   173     1.065086e+03     1.383866e+01
 * time: 202.57220196723938
   174     1.065086e+03     1.384608e+01
 * time: 203.67010498046875
   175     1.065086e+03     1.385678e+01
 * time: 204.8543348312378
   176     1.065084e+03     1.387273e+01
 * time: 205.98917984962463
   177     1.065081e+03     1.389858e+01
 * time: 207.19577383995056
   178     1.065073e+03     1.393897e+01
 * time: 208.28475403785706
   179     1.065052e+03     1.400288e+01
 * time: 209.45427203178406
   180     1.064997e+03     1.842052e+01
 * time: 210.57744097709656
   181     1.064853e+03     3.031555e+01
 * time: 211.78707599639893
   182     1.064477e+03     4.894991e+01
 * time: 212.97650504112244
   183     1.063505e+03     7.692533e+01
 * time: 214.1823079586029
   184     1.061069e+03     1.122192e+02
 * time: 215.38533186912537
   185     1.056257e+03     1.328367e+02
 * time: 216.71970582008362
   186     1.052013e+03     1.408303e+02
 * time: 218.09919381141663
   187     1.047713e+03     1.506293e+02
 * time: 219.50130200386047
   188     1.039359e+03     9.039176e+01
 * time: 220.93356585502625
   189     1.037334e+03     8.945366e+01
 * time: 222.2763228416443
   190     1.035766e+03     2.509488e+01
 * time: 223.68767499923706
   191     1.035510e+03     5.323250e+00
 * time: 225.24066281318665
   192     1.035401e+03     3.326584e+00
 * time: 226.6866638660431
   193     1.035202e+03     1.011867e+01
 * time: 228.01410698890686
   194     1.035130e+03     9.089057e+00
 * time: 229.40029788017273
   195     1.035055e+03     3.941605e+00
 * time: 230.67474603652954
   196     1.035048e+03     3.160942e+00
 * time: 231.99453902244568
   197     1.035046e+03     3.164670e+00
 * time: 233.27656602859497
   198     1.035046e+03     3.160486e+00
 * time: 234.60459995269775
   199     1.035046e+03     3.159428e+00
 * time: 235.84787583351135
   200     1.035046e+03     3.159138e+00
 * time: 237.25149393081665
   201     1.035046e+03     3.158158e+00
 * time: 238.49424290657043
   202     1.035046e+03     3.156875e+00
 * time: 239.76650881767273
   203     1.035046e+03     3.154403e+00
 * time: 241.0191400051117
   204     1.035045e+03     3.150100e+00
 * time: 242.37442803382874
   205     1.035043e+03     3.141854e+00
 * time: 243.71557188034058
   206     1.035038e+03     3.125468e+00
 * time: 245.09554290771484
   207     1.035025e+03     3.887763e+00
 * time: 246.44489288330078
   208     1.034991e+03     6.349435e+00
 * time: 247.80702686309814
   209     1.034902e+03     1.025429e+01
 * time: 249.11679792404175
   210     1.034678e+03     1.614807e+01
 * time: 250.53751802444458
   211     1.034143e+03     2.389258e+01
 * time: 251.87562203407288
   212     1.033018e+03     3.065325e+01
 * time: 253.20056986808777
   213     1.031157e+03     2.988569e+01
 * time: 254.49060893058777
   214     1.029913e+03     2.000449e+01
 * time: 255.87249898910522
   215     1.029264e+03     1.806624e+01
 * time: 257.23367285728455
   216     1.028739e+03     1.876674e+01
 * time: 258.6865060329437
   217     1.028529e+03     1.788313e+01
 * time: 260.28608798980713
   218     1.028407e+03     1.869928e+01
 * time: 261.70440101623535
   219     1.028318e+03     2.103440e+01
 * time: 263.15779399871826
   220     1.028259e+03     2.278116e+01
 * time: 264.562618970871
   221     1.028193e+03     2.168720e+01
 * time: 266.0227789878845
   222     1.028177e+03     2.174690e+01
 * time: 267.40657901763916
   223     1.028175e+03     2.195554e+01
 * time: 268.7896809577942
   224     1.028175e+03     2.193442e+01
 * time: 270.0408148765564
   225     1.028174e+03     2.202037e+01
 * time: 271.32663798332214
   226     1.028174e+03     2.201903e+01
 * time: 272.63357496261597
   227     1.028173e+03     2.200004e+01
 * time: 273.9634928703308
   228     1.028171e+03     2.197102e+01
 * time: 275.36669397354126
   229     1.028164e+03     2.191369e+01
 * time: 276.80560183525085
   230     1.028148e+03     2.181626e+01
 * time: 278.21602392196655
   231     1.028106e+03     2.163664e+01
 * time: 279.66400504112244
   232     1.027999e+03     3.038649e+01
 * time: 281.08665800094604
   233     1.027721e+03     5.035972e+01
 * time: 282.51730489730835
   234     1.027021e+03     8.114702e+01
 * time: 283.9128420352936
   235     1.025404e+03     1.233505e+02
 * time: 285.3278589248657
   236     1.022343e+03     1.646448e+02
 * time: 286.708988904953
   237     1.017556e+03     1.679173e+02
 * time: 288.10416889190674
   238     1.013261e+03     5.600035e+01
 * time: 289.4879689216614
   239     1.012953e+03     2.691504e+01
 * time: 290.9745988845825
   240     1.012758e+03     1.329779e+01
 * time: 292.3982479572296
   241     1.012704e+03     2.847118e+00
 * time: 293.7371709346771
   242     1.012699e+03     1.333414e+00
 * time: 295.260715007782
   243     1.012697e+03     9.979337e-01
 * time: 297.3005139827728
   244     1.012697e+03     9.969545e-01
 * time: 298.8340699672699
   245     1.012697e+03     9.971341e-01
 * time: 300.1969449520111
   246     1.012697e+03     9.971972e-01
 * time: 301.6381950378418
   247     1.012697e+03     9.972237e-01
 * time: 303.01478481292725
   248     1.012697e+03     9.972927e-01
 * time: 304.5128300189972
   249     1.012697e+03     9.973804e-01
 * time: 305.86771392822266
   250     1.012697e+03     9.975286e-01
 * time: 307.21462392807007
   251     1.012697e+03     9.977461e-01
 * time: 308.4917838573456
   252     1.012697e+03     9.980635e-01
 * time: 309.8630998134613
   253     1.012697e+03     9.984743e-01
 * time: 311.15561985969543
   254     1.012696e+03     9.988765e-01
 * time: 312.53824186325073
   255     1.012694e+03     9.988363e-01
 * time: 313.8297789096832
   256     1.012690e+03     1.474525e+00
 * time: 315.16408491134644
   257     1.012678e+03     2.326520e+00
 * time: 316.5202820301056
   258     1.012649e+03     3.510529e+00
 * time: 317.883083820343
   259     1.012590e+03     4.737310e+00
 * time: 319.14827394485474
   260     1.012497e+03     4.897808e+00
 * time: 320.4295380115509
   261     1.012416e+03     2.875911e+00
 * time: 321.68198800086975
   262     1.012389e+03     9.301211e-01
 * time: 323.0195360183716
   263     1.012386e+03     8.804644e-01
 * time: 324.33451795578003
   264     1.012386e+03     8.547350e-01
 * time: 325.61728501319885
   265     1.012386e+03     8.516621e-01
 * time: 326.8127980232239
   266     1.012386e+03     8.511780e-01
 * time: 328.1317489147186
   267     1.012386e+03     8.489977e-01
 * time: 329.40950989723206
   268     1.012386e+03     8.463566e-01
 * time: 330.7158989906311
   269     1.012386e+03     8.415231e-01
 * time: 331.9876148700714
   270     1.012386e+03     8.340037e-01
 * time: 333.2807819843292
   271     1.012386e+03     8.215489e-01
 * time: 334.4944579601288
   272     1.012386e+03     8.013257e-01
 * time: 335.81627798080444
   273     1.012385e+03     7.680260e-01
 * time: 337.11640882492065
   274     1.012383e+03     7.407512e-01
 * time: 338.476763010025
   275     1.012377e+03     7.469071e-01
 * time: 339.81145095825195
   276     1.012364e+03     1.103988e+00
 * time: 341.11222100257874
   277     1.012330e+03     1.710981e+00
 * time: 342.3841278553009
   278     1.012257e+03     2.412723e+00
 * time: 343.7205009460449
   279     1.012125e+03     2.705165e+00
 * time: 344.99701380729675
   280     1.011948e+03     1.883597e+00
 * time: 346.3488748073578
   281     1.011809e+03     8.215973e-01
 * time: 347.68405294418335
   282     1.011775e+03     9.067203e-01
 * time: 349.0686960220337
   283     1.011774e+03     9.565073e-01
 * time: 350.3848249912262
   284     1.011774e+03     9.628263e-01
 * time: 351.7163460254669
   285     1.011774e+03     9.654767e-01
 * time: 353.0205109119415
   286     1.011774e+03     9.654769e-01
 * time: 354.4735949039459
   287     1.011774e+03     9.656366e-01
 * time: 355.7786009311676
   288     1.011774e+03     9.656622e-01
 * time: 357.1124098300934
   289     1.011774e+03     9.656624e-01
 * time: 358.51899099349976
   290     1.011774e+03     9.656627e-01
 * time: 360.0175750255585
   291     1.011774e+03     9.660395e-01
 * time: 361.33866596221924
   292     1.011774e+03     9.661257e-01
 * time: 362.6686108112335
   293     1.011774e+03     9.663115e-01
 * time: 363.98257303237915
   294     1.011774e+03     9.662851e-01
 * time: 365.31491589546204
   295     1.011773e+03     9.654773e-01
 * time: 366.6513338088989
   296     1.011772e+03     9.623124e-01
 * time: 368.1098918914795
   297     1.011769e+03     9.521889e-01
 * time: 369.6161618232727
   298     1.011763e+03     9.239616e-01
 * time: 371.19315099716187
   299     1.011746e+03     9.242954e-01
 * time: 373.293741941452
   300     1.011711e+03     1.257052e+00
 * time: 375.54978489875793
   301     1.011651e+03     1.361278e+00
 * time: 377.30393385887146
   302     1.011591e+03     9.076916e-01
 * time: 378.6857178211212
   303     1.011568e+03     3.084752e-01
 * time: 380.0839250087738
   304     1.011565e+03     2.946404e-01
 * time: 381.5140948295593
   305     1.011565e+03     2.821273e-01
 * time: 382.9698758125305
   306     1.011565e+03     2.766260e-01
 * time: 384.28178787231445
   307     1.011565e+03     2.778010e-01
 * time: 385.66018080711365
   308     1.011565e+03     2.830281e-01
 * time: 386.9875729084015
   309     1.011565e+03     2.842619e-01
 * time: 388.4366409778595
   310     1.011565e+03     2.861093e-01
 * time: 389.75864601135254
   311     1.011565e+03     2.861138e-01
 * time: 391.143257856369
   312     1.011565e+03     2.867641e-01
 * time: 392.45394682884216
   313     1.011565e+03     2.867756e-01
 * time: 393.91198992729187
   314     1.011565e+03     2.884589e-01
 * time: 395.2377698421478
   315     1.011565e+03     2.897335e-01
 * time: 396.62285685539246
   316     1.011565e+03     2.927744e-01
 * time: 397.94901394844055
   317     1.011565e+03     2.967424e-01
 * time: 399.4180338382721
   318     1.011565e+03     3.027347e-01
 * time: 400.74935698509216
   319     1.011564e+03     3.099827e-01
 * time: 402.12612891197205
   320     1.011563e+03     3.155826e-01
 * time: 403.43930101394653
   321     1.011560e+03     3.082844e-01
 * time: 404.84043192863464
   322     1.011555e+03     2.625121e-01
 * time: 406.2319118976593
   323     1.011548e+03     2.136271e-01
 * time: 407.6485140323639
   324     1.011542e+03     1.165033e-01
 * time: 408.9716808795929
   325     1.011538e+03     3.914224e-02
 * time: 410.3546018600464
   326     1.011537e+03     3.083888e-02
 * time: 411.66406893730164
   327     1.011536e+03     2.766364e-02
 * time: 413.0256938934326
   328     1.011536e+03     1.873262e-02
 * time: 414.33494091033936
   329     1.011536e+03     5.453951e-03
 * time: 415.70627903938293
   330     1.011536e+03     4.882951e-03
 * time: 417.0085759162903
   331     1.011535e+03     4.331879e-03
 * time: 418.3646969795227
   332     1.011535e+03     2.711268e-03
 * time: 419.6513948440552
   333     1.011535e+03     4.474256e-04
 * time: 421.00161480903625
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: 5.793571472167969e-5
     1     4.804005e+05     8.280311e+05
 * time: 7.477102994918823
     2     3.595691e+05     6.035842e+05
 * time: 14.20088005065918
     3     1.691071e+05     2.702705e+05
 * time: 21.26009202003479
     4     9.193152e+04     1.451040e+05
 * time: 28.267995834350586
     5     4.768538e+04     6.630202e+04
 * time: 35.49591302871704
     6     2.904205e+04     3.416919e+04
 * time: 42.994040966033936
     7     1.840152e+04     1.671742e+04
 * time: 50.62706398963928
     8     1.280989e+04     1.022268e+04
 * time: 57.74581003189087
     9     9.713285e+03     9.298485e+03
 * time: 64.78573393821716
    10     7.628127e+03     8.330656e+03
 * time: 71.87553095817566
    11     6.236278e+03     7.340367e+03
 * time: 78.99835085868835
    12     5.271958e+03     6.303773e+03
 * time: 85.93837285041809
    13     4.411047e+03     4.994130e+03
 * time: 93.09591698646545
    14     3.732935e+03     3.538183e+03
 * time: 100.34382605552673
    15     3.399272e+03     2.450045e+03
 * time: 107.76529693603516
    16     3.302987e+03     1.848475e+03
 * time: 116.36773204803467
    17     3.291788e+03     1.642864e+03
 * time: 124.13626599311829
    18     3.290743e+03     1.594765e+03
 * time: 131.5236358642578
    19     3.289686e+03     1.562640e+03
 * time: 138.44833898544312
    20     3.286186e+03     1.496632e+03
 * time: 145.53403496742249
    21     3.277917e+03     1.402865e+03
 * time: 152.424654006958
    22     3.255958e+03     1.254296e+03
 * time: 159.40076398849487
    23     3.200979e+03     1.038621e+03
 * time: 166.37606000900269
    24     3.063520e+03     7.403602e+02
 * time: 173.53216099739075
    25     2.730315e+03     3.768701e+02
 * time: 180.51789784431458
    26     1.934186e+03     2.246121e+02
 * time: 187.11174893379211
    27     1.704399e+03     2.219826e+02
 * time: 207.3830909729004
    28     1.471184e+03     2.025825e+02
 * time: 243.10038304328918
    29     1.316789e+03     1.554715e+02
 * time: 271.69800305366516
    30     1.226363e+03     1.775721e+02
 * time: 279.2252838611603
    31     1.206626e+03     2.781030e+02
 * time: 286.22108483314514
    32     1.197830e+03     2.430603e+02
 * time: 293.89589285850525
    33     1.190342e+03     2.327966e+02
 * time: 301.04170083999634
    34     1.178784e+03     2.105878e+02
 * time: 308.3179819583893
    35     1.177853e+03     2.228653e+02
 * time: 315.1965889930725
    36     1.177764e+03     2.256182e+02
 * time: 322.1340448856354
    37     1.177746e+03     2.259425e+02
 * time: 329.45884799957275
    38     1.177670e+03     2.264699e+02
 * time: 336.3993408679962
    39     1.177500e+03     2.264641e+02
 * time: 343.32913088798523
    40     1.177034e+03     2.244207e+02
 * time: 349.971079826355
    41     1.175895e+03     2.160307e+02
 * time: 356.5888228416443
    42     1.173241e+03     1.906017e+02
 * time: 363.5140469074249
    43     1.168190e+03     1.332917e+02
 * time: 370.73134303092957
    44     1.161836e+03     5.291022e+01
 * time: 377.73172402381897
    45     1.158453e+03     6.595471e+01
 * time: 384.55436086654663
    46     1.157897e+03     6.411640e+01
 * time: 391.4858829975128
    47     1.157870e+03     6.098679e+01
 * time: 398.848669052124
    48     1.157868e+03     6.032481e+01
 * time: 406.0109009742737
    49     1.157864e+03     5.920991e+01
 * time: 413.4781210422516
    50     1.157856e+03     5.740417e+01
 * time: 420.1143798828125
    51     1.157832e+03     5.407031e+01
 * time: 426.4162678718567
    52     1.157774e+03     4.798188e+01
 * time: 432.9398410320282
    53     1.157628e+03     3.659183e+01
 * time: 439.49397683143616
    54     1.157299e+03     3.707862e+01
 * time: 446.3502309322357
    55     1.156690e+03     3.453183e+01
 * time: 453.38539600372314
    56     1.155959e+03     4.268473e+01
 * time: 460.09679102897644
    57     1.155557e+03     5.000309e+01
 * time: 466.87878799438477
    58     1.155472e+03     4.219411e+01
 * time: 473.5911400318146
    59     1.155463e+03     3.894982e+01
 * time: 480.45707082748413
    60     1.155462e+03     3.872028e+01
 * time: 487.3881139755249
    61     1.155458e+03     3.823435e+01
 * time: 494.3337490558624
    62     1.155450e+03     3.747328e+01
 * time: 500.9610879421234
    63     1.155426e+03     3.608641e+01
 * time: 507.5727210044861
    64     1.155365e+03     3.361168e+01
 * time: 514.1553828716278
    65     1.155207e+03     2.894844e+01
 * time: 520.8957798480988
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    67     1.153910e+03     4.539953e+01
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    69     1.150287e+03     7.473669e+01
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 * time: 555.5845458507538
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 * time: 562.4573459625244
    72     1.148932e+03     4.659470e+01
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    79     1.147881e+03     3.037355e+01
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    88     1.144281e+03     5.369030e+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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    94     1.140969e+03     4.309381e+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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   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
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   104     1.139452e+03     3.894107e+01
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   105     1.138224e+03     5.063688e+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
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   114     1.136604e+03     3.705604e+01
 * time: 849.9094970226288
   115     1.136418e+03     3.550032e+01
 * time: 856.9287269115448
   116     1.135940e+03     3.257323e+01
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   117     1.134744e+03     2.697619e+01
 * time: 869.6146769523621
   118     1.132016e+03     4.861352e+01
 * time: 876.00865483284
   119     1.127206e+03     6.900228e+01
 * time: 882.4315519332886
   120     1.122365e+03     5.795238e+01
 * time: 888.78635597229
   121     1.119760e+03     2.908163e+01
 * time: 895.0630879402161
   122     1.118972e+03     2.137897e+01
 * time: 901.3228008747101
   123     1.118870e+03     1.909827e+01
 * time: 907.5551149845123
   124     1.118863e+03     1.886936e+01
 * time: 913.7231669425964
   125     1.118862e+03     1.852746e+01
 * time: 919.9536299705505
   126     1.118861e+03     1.864804e+01
 * time: 926.0207929611206
   127     1.118861e+03     1.862831e+01
 * time: 932.1230659484863
   128     1.118860e+03     1.865947e+01
 * time: 938.2184588909149
   129     1.118859e+03     1.869546e+01
 * time: 944.3597228527069
   130     1.118856e+03     1.878621e+01
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   131     1.118850e+03     1.894171e+01
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   132     1.118834e+03     1.922062e+01
 * time: 962.8269338607788
   133     1.118794e+03     1.967952e+01
 * time: 968.9642698764801
   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
 * time: 1000.33251786232
   139     1.102031e+03     7.499119e+01
 * time: 1006.8579070568085
   140     1.090653e+03     6.250170e+01
 * time: 1013.6330850124359
   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
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   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
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   161     1.084458e+03     2.380805e+01
 * time: 1154.8254680633545
   162     1.083881e+03     2.403001e+01
 * time: 1161.6217608451843
   163     1.082583e+03     2.865879e+01
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   164     1.081739e+03     3.341843e+01
 * time: 1175.9617779254913
   165     1.078499e+03     5.054188e+01
 * time: 1183.535800933838
   166     1.075168e+03     6.966535e+01
 * time: 1190.4686889648438
   167     1.069624e+03     6.533378e+01
 * time: 1197.5330209732056
   168     1.068098e+03     3.856272e+01
 * time: 1204.2852709293365
   169     1.065571e+03     1.611062e+01
 * time: 1211.0892069339752
   170     1.065230e+03     1.527933e+01
 * time: 1217.8164949417114
   171     1.065088e+03     1.385503e+01
 * time: 1224.5282330513
   172     1.065086e+03     1.382742e+01
 * time: 1231.1653039455414
   173     1.065086e+03     1.383663e+01
 * time: 1237.719347000122
   174     1.065086e+03     1.384273e+01
 * time: 1244.3141379356384
   175     1.065086e+03     1.385109e+01
 * time: 1250.9322588443756
   176     1.065084e+03     1.386455e+01
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   177     1.065082e+03     1.388591e+01
 * time: 1264.1595029830933
   178     1.065074e+03     1.391915e+01
 * time: 1270.842260837555
   179     1.065053e+03     1.397116e+01
 * time: 1277.4702260494232
   180     1.065000e+03     1.742031e+01
 * time: 1284.166759967804
   181     1.064862e+03     2.855619e+01
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   182     1.064503e+03     4.585022e+01
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   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: 1324.580559015274
   187     1.048870e+03     1.445092e+02
 * time: 1331.3361270427704
   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
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   202     1.035046e+03     3.158994e+00
 * time: 1433.0484609603882
   203     1.035046e+03     3.157781e+00
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   204     1.035046e+03     3.155534e+00
 * time: 1446.325050830841
   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: 1473.0274319648743
   209     1.035012e+03     3.049096e+00
 * time: 1479.7888100147247
   210     1.034957e+03     3.798101e+00
 * time: 1486.5302548408508
   211     1.034816e+03     6.029607e+00
 * time: 1493.2755959033966
   212     1.034472e+03     9.154140e+00
 * time: 1500.0541770458221
   213     1.033706e+03     1.258550e+01
 * time: 1506.829735994339
   214     1.032296e+03     1.427962e+01
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   215     1.030400e+03     1.353250e+01
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   216     1.029562e+03     1.451097e+01
 * time: 1527.8525228500366
   217     1.028793e+03     1.534582e+01
 * time: 1534.665195941925
   218     1.028722e+03     2.309928e+01
 * time: 1541.5591349601746
   219     1.028409e+03     1.971948e+01
 * time: 1548.38991189003
   220     1.028326e+03     1.936501e+01
 * time: 1555.1787149906158
   221     1.028234e+03     2.040540e+01
 * time: 1562.0076110363007
   222     1.028219e+03     2.201826e+01
 * time: 1568.7916388511658
   223     1.028180e+03     2.177994e+01
 * time: 1575.58789396286
   224     1.028176e+03     2.186278e+01
 * time: 1582.3511340618134
   225     1.028175e+03     2.201733e+01
 * time: 1589.0519199371338
   226     1.028174e+03     2.197587e+01
 * time: 1595.7898778915405
   227     1.028174e+03     2.195893e+01
 * time: 1602.5038540363312
   228     1.028172e+03     2.186809e+01
 * time: 1609.1936249732971
   229     1.028169e+03     2.178132e+01
 * time: 1615.9349329471588
   230     1.028160e+03     2.162024e+01
 * time: 1622.7357950210571
   231     1.028139e+03     2.139041e+01
 * time: 1629.4710550308228
   232     1.028082e+03     2.100359e+01
 * time: 1636.247731924057
   233     1.027936e+03     2.808708e+01
 * time: 1643.0526330471039
   234     1.027561e+03     4.544763e+01
 * time: 1649.826231956482
   235     1.026628e+03     7.187426e+01
 * time: 1656.6105518341064
   236     1.024510e+03     1.065337e+02
 * time: 1663.3808720111847
   237     1.020621e+03     1.329824e+02
 * time: 1670.1320588588715
   238     1.015709e+03     1.045562e+02
 * time: 1676.8175840377808
   239     1.014365e+03     2.918128e+01
 * time: 1683.5658178329468
   240     1.013219e+03     1.803992e+01
 * time: 1690.7352068424225
   241     1.012886e+03     1.450533e+01
 * time: 1697.4801189899445
   242     1.012748e+03     9.039639e+00
 * time: 1704.1334309577942
   243     1.012704e+03     2.255849e+00
 * time: 1710.8311278820038
   244     1.012698e+03     1.107778e+00
 * time: 1717.562665939331
   245     1.012697e+03     9.968755e-01
 * time: 1724.1515300273895
   246     1.012697e+03     9.970599e-01
 * time: 1730.7131569385529
   247     1.012697e+03     9.971842e-01
 * time: 1737.2003688812256
   248     1.012697e+03     9.972169e-01
 * time: 1743.7274239063263
   249     1.012697e+03     9.972169e-01
 * time: 1750.5629479885101
   250     1.012697e+03     9.973243e-01
 * time: 1757.1029658317566
   251     1.012697e+03     9.973078e-01
 * time: 1763.6030659675598
   252     1.012697e+03     9.972048e-01
 * time: 1770.164715051651
   253     1.012697e+03     9.970536e-01
 * time: 1776.710932970047
   254     1.012697e+03     9.967402e-01
 * time: 1783.2858078479767
   255     1.012697e+03     9.961565e-01
 * time: 1789.8429749011993
   256     1.012696e+03     9.949478e-01
 * time: 1796.4373228549957
   257     1.012694e+03     1.312115e+00
 * time: 1803.016196012497
   258     1.012690e+03     2.118030e+00
 * time: 1809.6426079273224
   259     1.012679e+03     3.352490e+00
 * time: 1816.3227899074554
   260     1.012651e+03     5.076526e+00
 * time: 1822.902127981186
   261     1.012594e+03     6.891495e+00
 * time: 1829.5635678768158
   262     1.012501e+03     7.239074e+00
 * time: 1836.202301979065
   263     1.012418e+03     4.479611e+00
 * time: 1842.8770689964294
   264     1.012390e+03     1.267149e+00
 * time: 1849.507968902588
   265     1.012386e+03     8.840304e-01
 * time: 1856.1470608711243
   266     1.012386e+03     8.549097e-01
 * time: 1862.6924278736115
   267     1.012386e+03     8.516869e-01
 * time: 1869.1965990066528
   268     1.012386e+03     8.516869e-01
 * time: 1876.5875198841095
   269     1.012386e+03     8.516869e-01
 * time: 1884.451367855072
   270     1.012386e+03     8.516817e-01
 * time: 1891.5559208393097
   271     1.012386e+03     8.516817e-01
 * time: 1899.4079349040985
   272     1.012386e+03     8.516817e-01
 * time: 1907.3712079524994
   273     1.012386e+03     8.516817e-01
 * time: 1915.3160598278046
   274     1.012386e+03     8.516817e-01
 * time: 1923.8318769931793
   275     1.012386e+03     8.516817e-01
 * time: 1932.2589979171753
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: 6.604194641113281e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.0638132095336914
     2     3.595691e+05     6.035843e+05
 * time: 2.0554821491241455
     3     1.691071e+05     2.702705e+05
 * time: 3.0566630363464355
     4     9.193152e+04     1.451040e+05
 * time: 3.9885292053222656
     5     4.768539e+04     6.630202e+04
 * time: 4.974685192108154
     6     2.904205e+04     3.416919e+04
 * time: 5.890917062759399
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   246     1.012697e+03     9.971972e-01
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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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   254     1.012696e+03     9.988765e-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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   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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   289     1.011774e+03     9.656624e-01
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   290     1.011774e+03     9.656627e-01
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   292     1.011774e+03     9.661257e-01
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   293     1.011774e+03     9.663115e-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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   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
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   315     1.011565e+03     2.897335e-01
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   316     1.011565e+03     2.927744e-01
 * time: 311.5629379749298
   317     1.011565e+03     2.967424e-01
 * time: 312.58133602142334
   318     1.011565e+03     3.027347e-01
 * time: 313.62170600891113
   319     1.011564e+03     3.099827e-01
 * time: 314.6841471195221
   320     1.011563e+03     3.155826e-01
 * time: 315.7214241027832
   321     1.011560e+03     3.082844e-01
 * time: 316.79823899269104
   322     1.011555e+03     2.625121e-01
 * time: 317.8334891796112
   323     1.011548e+03     2.136271e-01
 * time: 318.92389011383057
   324     1.011542e+03     1.165033e-01
 * time: 319.9505581855774
   325     1.011538e+03     3.914224e-02
 * time: 320.99436211586
   326     1.011537e+03     3.083888e-02
 * time: 322.05453419685364
   327     1.011536e+03     2.766364e-02
 * time: 323.0910050868988
   328     1.011536e+03     1.873262e-02
 * time: 324.15885400772095
   329     1.011536e+03     5.453951e-03
 * time: 325.18024802207947
   330     1.011536e+03     4.882951e-03
 * time: 326.248740196228
   331     1.011535e+03     4.331879e-03
 * time: 327.2578241825104
   332     1.011535e+03     2.711268e-03
 * time: 328.283087015152
   333     1.011535e+03     4.474256e-04
 * time: 329.331463098526
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.