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: 0.02611708641052246
     1     4.816669e+05     8.316173e+05
 * time: 1.6927011013031006
     2     3.598373e+05     6.060150e+05
 * time: 2.319962978363037
     3     1.682124e+05     2.709072e+05
 * time: 2.967258930206299
     4     9.168914e+04     1.446832e+05
 * time: 3.546808958053589
     5     4.791138e+04     6.671463e+04
 * time: 4.163985967636108
     6     2.909104e+04     3.426663e+04
 * time: 4.722419023513794
     7     1.843737e+04     1.678953e+04
 * time: 5.321857929229736
     8     1.282447e+04     1.021463e+04
 * time: 5.921519041061401
     9     9.723361e+03     9.291650e+03
 * time: 6.5806050300598145
    10     7.635613e+03     8.325311e+03
 * time: 7.246090888977051
    11     6.243129e+03     7.337397e+03
 * time: 7.847722053527832
    12     5.279957e+03     6.304542e+03
 * time: 8.492357969284058
    13     4.422123e+03     5.001962e+03
 * time: 9.07016897201538
    14     3.744748e+03     3.550400e+03
 * time: 9.696433067321777
    15     3.409674e+03     2.461547e+03
 * time: 10.2811439037323
    16     3.312049e+03     1.856430e+03
 * time: 10.913904905319214
    17     3.300559e+03     1.648439e+03
 * time: 11.500211000442505
    18     3.299489e+03     1.599460e+03
 * time: 12.117809057235718
    19     3.298436e+03     1.567520e+03
 * time: 12.696063995361328
    20     3.294921e+03     1.501363e+03
 * time: 13.305933952331543
    21     3.286641e+03     1.407664e+03
 * time: 13.897119045257568
    22     3.264637e+03     1.259077e+03
 * time: 14.513629913330078
    23     3.209594e+03     1.043534e+03
 * time: 15.116370916366577
    24     3.072117e+03     7.454677e+02
 * time: 15.736212968826294
    25     2.739794e+03     3.818063e+02
 * time: 16.308451890945435
    26     1.949883e+03     2.246644e+02
 * time: 16.900930881500244
    27     1.722434e+03     2.224562e+02
 * time: 18.660545110702515
    28     1.479355e+03     2.042452e+02
 * time: 20.67979097366333
    29     1.310583e+03     1.531868e+02
 * time: 23.08453106880188
    30     1.224675e+03     1.756826e+02
 * time: 23.659393072128296
    31     1.204841e+03     2.738440e+02
 * time: 24.22037100791931
    32     1.196432e+03     2.403217e+02
 * time: 24.732858896255493
    33     1.189131e+03     2.301186e+02
 * time: 25.231034994125366
    34     1.177881e+03     2.083030e+02
 * time: 25.765413999557495
    35     1.176989e+03     2.201993e+02
 * time: 26.321758031845093
    36     1.176899e+03     2.230826e+02
 * time: 26.801843881607056
    37     1.176882e+03     2.233531e+02
 * time: 27.337141036987305
    38     1.176795e+03     2.238567e+02
 * time: 27.815344095230103
    39     1.176609e+03     2.236914e+02
 * time: 28.375654935836792
    40     1.176090e+03     2.210941e+02
 * time: 28.927793979644775
    41     1.174841e+03     2.112741e+02
 * time: 29.48563504219055
    42     1.171973e+03     1.825298e+02
 * time: 30.002063989639282
    43     1.166737e+03     1.210559e+02
 * time: 30.5693199634552
    44     1.160698e+03     4.450697e+01
 * time: 31.069344997406006
    45     1.157926e+03     6.411275e+01
 * time: 31.616962909698486
    46     1.157537e+03     6.121762e+01
 * time: 32.091017961502075
    47     1.157520e+03     5.867223e+01
 * time: 32.60736894607544
    48     1.157519e+03     5.814523e+01
 * time: 33.07475399971008
    49     1.157515e+03     5.686007e+01
 * time: 33.587517976760864
    50     1.157505e+03     5.497418e+01
 * time: 34.05543494224548
    51     1.157477e+03     5.129724e+01
 * time: 34.581542015075684
    52     1.157408e+03     4.463171e+01
 * time: 35.04270792007446
    53     1.157239e+03     3.279117e+01
 * time: 35.54072308540344
    54     1.156869e+03     3.681526e+01
 * time: 35.998672008514404
    55     1.156230e+03     3.198754e+01
 * time: 36.51727604866028
    56     1.155563e+03     4.576438e+01
 * time: 37.00059103965759
    57     1.155264e+03     4.832159e+01
 * time: 37.57898497581482
    58     1.155212e+03     4.052175e+01
 * time: 38.06172299385071
    59     1.155207e+03     3.797125e+01
 * time: 38.58347511291504
    60     1.155206e+03     3.778452e+01
 * time: 39.05156707763672
    61     1.155201e+03     3.723003e+01
 * time: 39.559499979019165
    62     1.155190e+03     3.642375e+01
 * time: 40.0743510723114
    63     1.155159e+03     3.487535e+01
 * time: 40.61317491531372
    64     1.155079e+03     3.209060e+01
 * time: 41.096494913101196
    65     1.154875e+03     2.670227e+01
 * time: 41.61755394935608
    66     1.154375e+03     2.366165e+01
 * time: 42.11773991584778
    67     1.153278e+03     5.499525e+01
 * time: 42.618874073028564
    68     1.151457e+03     7.903833e+01
 * time: 43.09328508377075
    69     1.149737e+03     6.771227e+01
 * time: 43.587100982666016
    70     1.148983e+03     5.387708e+01
 * time: 44.05850791931152
    71     1.148872e+03     4.683950e+01
 * time: 44.543018102645874
    72     1.148868e+03     4.512036e+01
 * time: 44.99015188217163
    73     1.148866e+03     4.489706e+01
 * time: 45.45267295837402
    74     1.148857e+03     4.426314e+01
 * time: 45.896398067474365
    75     1.148838e+03     4.321161e+01
 * time: 46.36517691612244
    76     1.148785e+03     4.095232e+01
 * time: 46.809650897979736
    77     1.148652e+03     3.780572e+01
 * time: 47.271559953689575
    78     1.148325e+03     3.474239e+01
 * time: 47.72371006011963
    79     1.147599e+03     3.678957e+01
 * time: 48.236716985702515
    80     1.146330e+03     5.134277e+01
 * time: 48.7106659412384
    81     1.144979e+03     6.444063e+01
 * time: 49.19164299964905
    82     1.144325e+03     7.574751e+01
 * time: 49.65359592437744
    83     1.144210e+03     6.968296e+01
 * time: 50.11248302459717
    84     1.144204e+03     6.648234e+01
 * time: 50.5857629776001
    85     1.144202e+03     6.560999e+01
 * time: 51.005887031555176
    86     1.144194e+03     6.305667e+01
 * time: 51.495179891586304
    87     1.144176e+03     5.956959e+01
 * time: 51.92906999588013
    88     1.144127e+03     5.307828e+01
 * time: 52.41878604888916
    89     1.144005e+03     4.218776e+01
 * time: 52.89321708679199
    90     1.143708e+03     4.120261e+01
 * time: 53.39156103134155
    91     1.143074e+03     3.675516e+01
 * time: 53.848143100738525
    92     1.142052e+03     3.855439e+01
 * time: 54.36059594154358
    93     1.141101e+03     5.193197e+01
 * time: 54.79287004470825
    94     1.140701e+03     4.606858e+01
 * time: 55.29709196090698
    95     1.140646e+03     4.857736e+01
 * time: 55.73552393913269
    96     1.140643e+03     4.817596e+01
 * time: 56.23041796684265
    97     1.140642e+03     4.793197e+01
 * time: 56.64785599708557
    98     1.140637e+03     4.723245e+01
 * time: 57.13870406150818
    99     1.140626e+03     4.619695e+01
 * time: 57.57896399497986
   100     1.140597e+03     4.418761e+01
 * time: 58.08202600479126
   101     1.140523e+03     4.050986e+01
 * time: 58.54635310173035
   102     1.140334e+03     3.335334e+01
 * time: 59.06259489059448
   103     1.139892e+03     2.982120e+01
 * time: 59.5269410610199
   104     1.138994e+03     4.360237e+01
 * time: 60.038482904434204
   105     1.137687e+03     5.246892e+01
 * time: 60.49608492851257
   106     1.136679e+03     4.825586e+01
 * time: 60.9785521030426
   107     1.136391e+03     4.467499e+01
 * time: 61.425689935684204
   108     1.136364e+03     3.961591e+01
 * time: 61.912643909454346
   109     1.136362e+03     3.951402e+01
 * time: 62.365931034088135
   110     1.136360e+03     3.943358e+01
 * time: 62.84543204307556
   111     1.136355e+03     3.923965e+01
 * time: 63.28848695755005
   112     1.136344e+03     3.894674e+01
 * time: 63.76533889770508
   113     1.136314e+03     3.841518e+01
 * time: 64.2070300579071
   114     1.136235e+03     3.748824e+01
 * time: 64.70638608932495
   115     1.136029e+03     3.577210e+01
 * time: 65.1598129272461
   116     1.135499e+03     3.252332e+01
 * time: 65.64731097221375
   117     1.134182e+03     2.630844e+01
 * time: 66.09335088729858
   118     1.131238e+03     4.869163e+01
 * time: 66.5899178981781
   119     1.126345e+03     6.537856e+01
 * time: 67.06660389900208
   120     1.121879e+03     5.123373e+01
 * time: 67.57467794418335
   121     1.119645e+03     2.457316e+01
 * time: 68.06685590744019
   122     1.119031e+03     2.124721e+01
 * time: 68.57914400100708
   123     1.118958e+03     1.857508e+01
 * time: 69.04415702819824
   124     1.118953e+03     1.896595e+01
 * time: 69.51784610748291
   125     1.118952e+03     1.849021e+01
 * time: 69.97680711746216
   126     1.118952e+03     1.852941e+01
 * time: 70.46055793762207
   127     1.118951e+03     1.856812e+01
 * time: 70.91250801086426
   128     1.118951e+03     1.862403e+01
 * time: 71.39392709732056
   129     1.118949e+03     1.871484e+01
 * time: 71.8651750087738
   130     1.118946e+03     1.886332e+01
 * time: 72.36351299285889
   131     1.118939e+03     1.910217e+01
 * time: 72.84254598617554
   132     1.118922e+03     1.948050e+01
 * time: 73.34192299842834
   133     1.118879e+03     2.006903e+01
 * time: 73.8318419456482
   134     1.118769e+03     2.097106e+01
 * time: 74.33555293083191
   135     1.118486e+03     2.231801e+01
 * time: 74.82484793663025
   136     1.117755e+03     3.049039e+01
 * time: 75.31615400314331
   137     1.115883e+03     5.345487e+01
 * time: 75.8330340385437
   138     1.111258e+03     8.489274e+01
 * time: 76.29019093513489
   139     1.101149e+03     7.643749e+01
 * time: 76.80460810661316
   140     1.090612e+03     5.802659e+01
 * time: 77.30336809158325
   141     1.087971e+03     5.776554e+01
 * time: 77.8821210861206
   142     1.087205e+03     1.148234e+02
 * time: 78.39083504676819
   143     1.085558e+03     4.724872e+01
 * time: 78.94654893875122
   144     1.085271e+03     2.018848e+01
 * time: 79.53140592575073
   145     1.085124e+03     2.004201e+01
 * time: 80.11560010910034
   146     1.085089e+03     2.025616e+01
 * time: 80.66102409362793
   147     1.085081e+03     2.049682e+01
 * time: 81.2650499343872
   148     1.085073e+03     2.086075e+01
 * time: 81.83065295219421
   149     1.085070e+03     2.095706e+01
 * time: 82.41697311401367
   150     1.085063e+03     2.094978e+01
 * time: 83.0330240726471
   151     1.085062e+03     2.093855e+01
 * time: 83.62757897377014
   152     1.085062e+03     2.093439e+01
 * time: 84.33828902244568
   153     1.085062e+03     2.093335e+01
 * time: 85.02231097221375
   154     1.085062e+03     2.093252e+01
 * time: 85.75088500976562
   155     1.085062e+03     2.093227e+01
 * time: 86.53235697746277
   156     1.085062e+03     2.093208e+01
 * time: 87.28588199615479
   157     1.085062e+03     2.093187e+01
 * time: 88.07127809524536
   158     1.085062e+03     2.093181e+01
 * time: 88.8927481174469
   159     1.085062e+03     2.093175e+01
 * time: 89.70535898208618
   160     1.085062e+03     2.093172e+01
 * time: 90.62055897712708
   161     1.085062e+03     2.046653e+01
 * time: 91.21287393569946
   162     1.085061e+03     2.045920e+01
 * time: 91.91516494750977
   163     1.085056e+03     2.021786e+01
 * time: 92.48618793487549
   164     1.085055e+03     2.018381e+01
 * time: 93.17255306243896
   165     1.085043e+03     1.997498e+01
 * time: 93.72569990158081
   166     1.085029e+03     1.961695e+01
 * time: 94.30105209350586
   167     1.084997e+03     1.929634e+01
 * time: 94.83890390396118
   168     1.084881e+03     1.855033e+01
 * time: 95.40808510780334
   169     1.084633e+03     1.799616e+01
 * time: 95.96174192428589
   170     1.083831e+03     1.856893e+01
 * time: 96.5353569984436
   171     1.081240e+03     2.069096e+01
 * time: 97.09978604316711
   172     1.072465e+03     3.262436e+01
 * time: 97.74869298934937
   173     1.072446e+03     6.686587e+01
 * time: 98.41009998321533
   174     1.068493e+03     7.417042e+01
 * time: 99.02554488182068
   175     1.067469e+03     7.109936e+01
 * time: 99.79062604904175
   176     1.065854e+03     2.774005e+01
 * time: 100.43494391441345
   177     1.065215e+03     1.328875e+01
 * time: 101.10913801193237
   178     1.065128e+03     1.390400e+01
 * time: 101.76535105705261
   179     1.065120e+03     1.383172e+01
 * time: 102.42727994918823
   180     1.065120e+03     1.384995e+01
 * time: 103.04243898391724
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: 7.510185241699219e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.3474841117858887
     2     3.595691e+05     6.035843e+05
 * time: 2.534959077835083
     3     1.691071e+05     2.702705e+05
 * time: 3.717705011367798
     4     9.193152e+04     1.451040e+05
 * time: 4.92848014831543
     5     4.768539e+04     6.630202e+04
 * time: 6.130375146865845
     6     2.904205e+04     3.416919e+04
 * time: 7.328541994094849
     7     1.840153e+04     1.671742e+04
 * time: 8.48523998260498
     8     1.280989e+04     1.022268e+04
 * time: 9.695870161056519
     9     9.713287e+03     9.298485e+03
 * time: 10.851838111877441
    10     7.628127e+03     8.330656e+03
 * time: 12.051741123199463
    11     6.236278e+03     7.340367e+03
 * time: 13.207720041275024
    12     5.271958e+03     6.303773e+03
 * time: 14.375622987747192
    13     4.411047e+03     4.994130e+03
 * time: 15.515275955200195
    14     3.732936e+03     3.538184e+03
 * time: 16.621971130371094
    15     3.399273e+03     2.450045e+03
 * time: 17.73664402961731
    16     3.302987e+03     1.848476e+03
 * time: 18.80033802986145
    17     3.291788e+03     1.642864e+03
 * time: 19.933995008468628
    18     3.290743e+03     1.594765e+03
 * time: 20.97487711906433
    19     3.289687e+03     1.562641e+03
 * time: 22.053393125534058
    20     3.286186e+03     1.496632e+03
 * time: 23.054267168045044
    21     3.277917e+03     1.402865e+03
 * time: 24.133937120437622
    22     3.255959e+03     1.254297e+03
 * time: 25.18813705444336
    23     3.200980e+03     1.038621e+03
 * time: 26.219139099121094
    24     3.063521e+03     7.403605e+02
 * time: 27.16952896118164
    25     2.730316e+03     3.768701e+02
 * time: 28.166651964187622
    26     1.934186e+03     2.246121e+02
 * time: 29.047192096710205
    27     1.704399e+03     2.219826e+02
 * time: 32.77497100830078
    28     1.471183e+03     2.025826e+02
 * time: 39.22922110557556
    29     1.316789e+03     1.554714e+02
 * time: 44.90329718589783
    30     1.226363e+03     1.775666e+02
 * time: 45.944130182266235
    31     1.206626e+03     2.780980e+02
 * time: 46.81198215484619
    32     1.197830e+03     2.430550e+02
 * time: 47.71193599700928
    33     1.190342e+03     2.327905e+02
 * time: 48.57404804229736
    34     1.178783e+03     2.105809e+02
 * time: 49.50160813331604
    35     1.177853e+03     2.228585e+02
 * time: 50.375293016433716
    36     1.177763e+03     2.256116e+02
 * time: 51.34533905982971
    37     1.177745e+03     2.259359e+02
 * time: 52.28489708900452
    38     1.177670e+03     2.264633e+02
 * time: 53.25929117202759
    39     1.177500e+03     2.264574e+02
 * time: 54.24158501625061
    40     1.177033e+03     2.244136e+02
 * time: 55.28711915016174
    41     1.175894e+03     2.160226e+02
 * time: 56.263370990753174
    42     1.173240e+03     1.905912e+02
 * time: 57.19015312194824
    43     1.168189e+03     1.332783e+02
 * time: 58.121915102005005
    44     1.161835e+03     5.289952e+01
 * time: 59.07906198501587
    45     1.158453e+03     6.595387e+01
 * time: 60.019392013549805
    46     1.157897e+03     6.411419e+01
 * time: 61.00095009803772
    47     1.157870e+03     6.098509e+01
 * time: 61.95716595649719
    48     1.157868e+03     6.032325e+01
 * time: 62.922281980514526
    49     1.157864e+03     5.920812e+01
 * time: 63.877477169036865
    50     1.157856e+03     5.740225e+01
 * time: 64.8551549911499
    51     1.157832e+03     5.406795e+01
 * time: 65.82625603675842
    52     1.157774e+03     4.797879e+01
 * time: 66.82449412345886
    53     1.157628e+03     3.658732e+01
 * time: 67.79346299171448
    54     1.157299e+03     3.707874e+01
 * time: 68.8206729888916
    55     1.156690e+03     3.452947e+01
 * time: 69.74993896484375
    56     1.155959e+03     4.269004e+01
 * time: 70.74152207374573
    57     1.155557e+03     5.000322e+01
 * time: 71.72419309616089
    58     1.155472e+03     4.219349e+01
 * time: 72.62027597427368
    59     1.155463e+03     3.895049e+01
 * time: 73.53994417190552
    60     1.155462e+03     3.872099e+01
 * time: 74.3825261592865
    61     1.155458e+03     3.823491e+01
 * time: 75.28940105438232
    62     1.155450e+03     3.747370e+01
 * time: 76.14975619316101
    63     1.155426e+03     3.608648e+01
 * time: 77.07763314247131
    64     1.155365e+03     3.361118e+01
 * time: 77.93202996253967
    65     1.155207e+03     2.894678e+01
 * time: 78.84740018844604
    66     1.154814e+03     2.504284e+01
 * time: 79.70571708679199
    67     1.153910e+03     4.541126e+01
 * time: 80.62178111076355
    68     1.152230e+03     7.426044e+01
 * time: 81.48539304733276
    69     1.150286e+03     7.473044e+01
 * time: 82.40299010276794
    70     1.149188e+03     5.605967e+01
 * time: 83.25646996498108
    71     1.148944e+03     4.801123e+01
 * time: 84.15591716766357
    72     1.148931e+03     4.659478e+01
 * time: 85.03090405464172
    73     1.148929e+03     4.635180e+01
 * time: 85.98858714103699
    74     1.148922e+03     4.568579e+01
 * time: 86.90504503250122
    75     1.148907e+03     4.466128e+01
 * time: 87.93957805633545
    76     1.148864e+03     4.249742e+01
 * time: 88.91295409202576
    77     1.148758e+03     3.824840e+01
 * time: 89.93128895759583
    78     1.148490e+03     3.552459e+01
 * time: 90.89270496368408
    79     1.147881e+03     3.038181e+01
 * time: 91.88046097755432
    80     1.146736e+03     4.467281e+01
 * time: 92.81267499923706
    81     1.145347e+03     5.929158e+01
 * time: 93.78445601463318
    82     1.144535e+03     7.572784e+01
 * time: 94.71028208732605
    83     1.144354e+03     7.034986e+01
 * time: 95.68195104598999
    84     1.144341e+03     6.591584e+01
 * time: 96.59441900253296
    85     1.144340e+03     6.496353e+01
 * time: 97.53505897521973
    86     1.144333e+03     6.268267e+01
 * time: 98.43770098686218
    87     1.144320e+03     5.950121e+01
 * time: 99.3805251121521
    88     1.144281e+03     5.368613e+01
 * time: 100.2833821773529
    89     1.144186e+03     4.393956e+01
 * time: 101.23476505279541
    90     1.143949e+03     4.102324e+01
 * time: 102.15888214111328
    91     1.143423e+03     3.752005e+01
 * time: 103.16964411735535
    92     1.142497e+03     3.278884e+01
 * time: 104.14113306999207
    93     1.141489e+03     5.217814e+01
 * time: 105.18705010414124
    94     1.140969e+03     4.310032e+01
 * time: 106.15299105644226
    95     1.140872e+03     4.759545e+01
 * time: 107.1604061126709
    96     1.140867e+03     4.727034e+01
 * time: 108.11766314506531
    97     1.140866e+03     4.704365e+01
 * time: 109.10540509223938
    98     1.140862e+03     4.637923e+01
 * time: 110.06973505020142
    99     1.140853e+03     4.542582e+01
 * time: 111.05847597122192
   100     1.140829e+03     4.357378e+01
 * time: 112.01939797401428
   101     1.140767e+03     4.023060e+01
 * time: 113.01356315612793
   102     1.140610e+03     3.376323e+01
 * time: 113.98938798904419
   103     1.140237e+03     2.857028e+01
 * time: 114.96671199798584
   104     1.139451e+03     3.895699e+01
 * time: 115.93339896202087
   105     1.138223e+03     5.064036e+01
 * time: 116.92458295822144
   106     1.137140e+03     4.763314e+01
 * time: 117.87785601615906
   107     1.136763e+03     4.604968e+01
 * time: 118.84796404838562
   108     1.136720e+03     3.993479e+01
 * time: 119.80368900299072
   109     1.136717e+03     3.890565e+01
 * time: 120.7070369720459
   110     1.136716e+03     3.883645e+01
 * time: 121.64087009429932
   111     1.136712e+03     3.865517e+01
 * time: 122.62205815315247
   112     1.136702e+03     3.838833e+01
 * time: 123.59617400169373
   113     1.136674e+03     3.789981e+01
 * time: 124.65812611579895
   114     1.136603e+03     3.705558e+01
 * time: 125.61389803886414
   115     1.136418e+03     3.549940e+01
 * time: 126.5742540359497
   116     1.135940e+03     3.257136e+01
 * time: 127.5256609916687
   117     1.134743e+03     2.697248e+01
 * time: 128.50862216949463
   118     1.132014e+03     4.863416e+01
 * time: 129.4931299686432
   119     1.127203e+03     6.901202e+01
 * time: 130.50301504135132
   120     1.122363e+03     5.794311e+01
 * time: 131.51960611343384
   121     1.119759e+03     2.906766e+01
 * time: 132.49630904197693
   122     1.118972e+03     2.137843e+01
 * time: 133.541846036911
   123     1.118870e+03     1.909639e+01
 * time: 134.48754811286926
   124     1.118863e+03     1.887024e+01
 * time: 135.48727416992188
   125     1.118862e+03     1.852691e+01
 * time: 136.41160607337952
   126     1.118861e+03     1.864783e+01
 * time: 137.35923099517822
   127     1.118861e+03     1.862870e+01
 * time: 138.22820115089417
   128     1.118860e+03     1.866000e+01
 * time: 139.15695714950562
   129     1.118859e+03     1.869625e+01
 * time: 140.0890290737152
   130     1.118856e+03     1.878717e+01
 * time: 141.1207160949707
   131     1.118850e+03     1.894283e+01
 * time: 142.07358717918396
   132     1.118834e+03     1.922180e+01
 * time: 143.06053018569946
   133     1.118794e+03     1.968060e+01
 * time: 143.98213005065918
   134     1.118693e+03     2.042353e+01
 * time: 144.9784071445465
   135     1.118431e+03     2.156998e+01
 * time: 145.90903306007385
   136     1.117751e+03     2.596600e+01
 * time: 146.9029619693756
   137     1.116007e+03     4.716393e+01
 * time: 147.8466341495514
   138     1.111677e+03     7.744992e+01
 * time: 148.8613531589508
   139     1.102024e+03     7.496268e+01
 * time: 149.85454106330872
   140     1.090649e+03     6.246756e+01
 * time: 150.9897689819336
   141     1.087843e+03     8.941834e+01
 * time: 152.04535698890686
   142     1.087400e+03     1.188539e+02
 * time: 153.144198179245
   143     1.085563e+03     6.082023e+01
 * time: 154.2367341518402
   144     1.085185e+03     1.947062e+01
 * time: 155.4311580657959
   145     1.085039e+03     1.958695e+01
 * time: 156.49481010437012
   146     1.084989e+03     1.989918e+01
 * time: 157.57253909111023
   147     1.084983e+03     2.009395e+01
 * time: 158.61629796028137
   148     1.084973e+03     2.065564e+01
 * time: 159.69237899780273
   149     1.084971e+03     2.080010e+01
 * time: 160.75446009635925
   150     1.084971e+03     2.084232e+01
 * time: 161.83118605613708
   151     1.084970e+03     2.084506e+01
 * time: 162.90652513504028
   152     1.084970e+03     2.084321e+01
 * time: 163.93244695663452
   153     1.084968e+03     2.084737e+01
 * time: 165.03135299682617
   154     1.084963e+03     2.088045e+01
 * time: 166.1019971370697
   155     1.084954e+03     2.098376e+01
 * time: 167.17755699157715
   156     1.084935e+03     2.121619e+01
 * time: 168.2111291885376
   157     1.084904e+03     2.160158e+01
 * time: 169.31435704231262
   158     1.084864e+03     2.208328e+01
 * time: 170.3643810749054
   159     1.084807e+03     2.260004e+01
 * time: 171.4704830646515
   160     1.084700e+03     2.319870e+01
 * time: 172.52721214294434
   161     1.084457e+03     2.382124e+01
 * time: 173.64124703407288
   162     1.083882e+03     2.406095e+01
 * time: 174.73914003372192
   163     1.082589e+03     2.849219e+01
 * time: 175.93204307556152
   164     1.081660e+03     3.379635e+01
 * time: 177.1937861442566
   165     1.078576e+03     4.983497e+01
 * time: 178.52061915397644
   166     1.075004e+03     6.870527e+01
 * time: 179.72764706611633
   167     1.069377e+03     6.309176e+01
 * time: 180.94233918190002
   168     1.067978e+03     3.811944e+01
 * time: 182.10993099212646
   169     1.065604e+03     1.825143e+01
 * time: 183.23571515083313
   170     1.065251e+03     1.739631e+01
 * time: 184.42520809173584
   171     1.065088e+03     1.384781e+01
 * time: 185.53869104385376
   172     1.065086e+03     1.382386e+01
 * time: 186.69584918022156
   173     1.065086e+03     1.383866e+01
 * time: 187.77788400650024
   174     1.065086e+03     1.384608e+01
 * time: 188.9050669670105
   175     1.065086e+03     1.385678e+01
 * time: 189.9957971572876
   176     1.065084e+03     1.387273e+01
 * time: 191.13106513023376
   177     1.065081e+03     1.389858e+01
 * time: 192.22824001312256
   178     1.065073e+03     1.393897e+01
 * time: 193.39007306098938
   179     1.065052e+03     1.400288e+01
 * time: 194.50037002563477
   180     1.064997e+03     1.842052e+01
 * time: 195.7239441871643
   181     1.064853e+03     3.031555e+01
 * time: 196.9201500415802
   182     1.064477e+03     4.894991e+01
 * time: 198.15338397026062
   183     1.063505e+03     7.692533e+01
 * time: 199.29648613929749
   184     1.061069e+03     1.122192e+02
 * time: 200.48354601860046
   185     1.056257e+03     1.328367e+02
 * time: 201.71900010108948
   186     1.052013e+03     1.408303e+02
 * time: 203.04372596740723
   187     1.047713e+03     1.506293e+02
 * time: 204.38660097122192
   188     1.039359e+03     9.039176e+01
 * time: 205.74722599983215
   189     1.037334e+03     8.945366e+01
 * time: 207.14332008361816
   190     1.035766e+03     2.509488e+01
 * time: 208.5836660861969
   191     1.035510e+03     5.323250e+00
 * time: 210.0991370677948
   192     1.035401e+03     3.326584e+00
 * time: 211.45446014404297
   193     1.035202e+03     1.011867e+01
 * time: 212.80799508094788
   194     1.035130e+03     9.089057e+00
 * time: 214.09160900115967
   195     1.035055e+03     3.941605e+00
 * time: 215.39764308929443
   196     1.035048e+03     3.160942e+00
 * time: 216.62513303756714
   197     1.035046e+03     3.164670e+00
 * time: 217.92667412757874
   198     1.035046e+03     3.160486e+00
 * time: 219.22040796279907
   199     1.035046e+03     3.159428e+00
 * time: 220.54391312599182
   200     1.035046e+03     3.159138e+00
 * time: 221.83150506019592
   201     1.035046e+03     3.158158e+00
 * time: 223.0769441127777
   202     1.035046e+03     3.156875e+00
 * time: 224.29384899139404
   203     1.035046e+03     3.154403e+00
 * time: 225.54087114334106
   204     1.035045e+03     3.150100e+00
 * time: 226.7567789554596
   205     1.035043e+03     3.141854e+00
 * time: 228.040922164917
   206     1.035038e+03     3.125468e+00
 * time: 229.27934002876282
   207     1.035025e+03     3.887763e+00
 * time: 230.5434501171112
   208     1.034991e+03     6.349435e+00
 * time: 231.78477811813354
   209     1.034902e+03     1.025429e+01
 * time: 233.0883550643921
   210     1.034678e+03     1.614807e+01
 * time: 234.359521150589
   211     1.034143e+03     2.389258e+01
 * time: 235.64566111564636
   212     1.033018e+03     3.065325e+01
 * time: 236.89837908744812
   213     1.031157e+03     2.988569e+01
 * time: 238.19138312339783
   214     1.029913e+03     2.000449e+01
 * time: 239.4906210899353
   215     1.029264e+03     1.806624e+01
 * time: 240.78755903244019
   216     1.028739e+03     1.876674e+01
 * time: 242.11535596847534
   217     1.028529e+03     1.788313e+01
 * time: 243.49202919006348
   218     1.028407e+03     1.869928e+01
 * time: 244.86155796051025
   219     1.028318e+03     2.103440e+01
 * time: 246.16638112068176
   220     1.028259e+03     2.278116e+01
 * time: 247.52951502799988
   221     1.028193e+03     2.168720e+01
 * time: 248.79927515983582
   222     1.028177e+03     2.174690e+01
 * time: 250.12381100654602
   223     1.028175e+03     2.195554e+01
 * time: 251.4384319782257
   224     1.028175e+03     2.193442e+01
 * time: 252.75515699386597
   225     1.028174e+03     2.202037e+01
 * time: 254.01793313026428
   226     1.028174e+03     2.201903e+01
 * time: 255.36856508255005
   227     1.028173e+03     2.200004e+01
 * time: 256.68689918518066
   228     1.028171e+03     2.197102e+01
 * time: 258.09642910957336
   229     1.028164e+03     2.191369e+01
 * time: 259.4621510505676
   230     1.028148e+03     2.181626e+01
 * time: 260.86881613731384
   231     1.028106e+03     2.163664e+01
 * time: 262.1945970058441
   232     1.027999e+03     3.038649e+01
 * time: 263.53837513923645
   233     1.027721e+03     5.035972e+01
 * time: 264.87301898002625
   234     1.027021e+03     8.114702e+01
 * time: 266.2224850654602
   235     1.025404e+03     1.233505e+02
 * time: 267.5183789730072
   236     1.022343e+03     1.646448e+02
 * time: 268.8187379837036
   237     1.017556e+03     1.679173e+02
 * time: 270.0867409706116
   238     1.013261e+03     5.600035e+01
 * time: 271.4029791355133
   239     1.012953e+03     2.691504e+01
 * time: 272.81420397758484
   240     1.012758e+03     1.329779e+01
 * time: 274.0690031051636
   241     1.012704e+03     2.847118e+00
 * time: 275.37173414230347
   242     1.012699e+03     1.333414e+00
 * time: 276.6093831062317
   243     1.012697e+03     9.979337e-01
 * time: 277.9058690071106
   244     1.012697e+03     9.969545e-01
 * time: 279.125529050827
   245     1.012697e+03     9.971341e-01
 * time: 280.39104318618774
   246     1.012697e+03     9.971972e-01
 * time: 281.5947790145874
   247     1.012697e+03     9.972237e-01
 * time: 282.8454451560974
   248     1.012697e+03     9.972927e-01
 * time: 284.0457501411438
   249     1.012697e+03     9.973804e-01
 * time: 285.31747603416443
   250     1.012697e+03     9.975286e-01
 * time: 286.54048800468445
   251     1.012697e+03     9.977461e-01
 * time: 287.84970116615295
   252     1.012697e+03     9.980635e-01
 * time: 289.0320200920105
   253     1.012697e+03     9.984743e-01
 * time: 290.2694561481476
   254     1.012696e+03     9.988765e-01
 * time: 291.4479660987854
   255     1.012694e+03     9.988363e-01
 * time: 292.6988320350647
   256     1.012690e+03     1.474525e+00
 * time: 293.89004015922546
   257     1.012678e+03     2.326520e+00
 * time: 295.13222312927246
   258     1.012649e+03     3.510529e+00
 * time: 296.31842517852783
   259     1.012590e+03     4.737310e+00
 * time: 297.5577030181885
   260     1.012497e+03     4.897808e+00
 * time: 298.7741370201111
   261     1.012416e+03     2.875911e+00
 * time: 300.03369402885437
   262     1.012389e+03     9.301211e-01
 * time: 301.2374610900879
   263     1.012386e+03     8.804644e-01
 * time: 302.47693514823914
   264     1.012386e+03     8.547350e-01
 * time: 303.6633040904999
   265     1.012386e+03     8.516621e-01
 * time: 304.89858317375183
   266     1.012386e+03     8.511780e-01
 * time: 306.0610270500183
   267     1.012386e+03     8.489977e-01
 * time: 307.28275418281555
   268     1.012386e+03     8.463566e-01
 * time: 308.4570310115814
   269     1.012386e+03     8.415231e-01
 * time: 309.68901896476746
   270     1.012386e+03     8.340037e-01
 * time: 310.92344212532043
   271     1.012386e+03     8.215489e-01
 * time: 312.15370512008667
   272     1.012386e+03     8.013257e-01
 * time: 313.33257508277893
   273     1.012385e+03     7.680260e-01
 * time: 314.562420129776
   274     1.012383e+03     7.407512e-01
 * time: 315.75735807418823
   275     1.012377e+03     7.469071e-01
 * time: 316.99012899398804
   276     1.012364e+03     1.103988e+00
 * time: 318.22072315216064
   277     1.012330e+03     1.710981e+00
 * time: 319.46046209335327
   278     1.012257e+03     2.412723e+00
 * time: 320.78011202812195
   279     1.012125e+03     2.705165e+00
 * time: 322.14967012405396
   280     1.011948e+03     1.883597e+00
 * time: 323.4102909564972
   281     1.011809e+03     8.215973e-01
 * time: 324.71212100982666
   282     1.011775e+03     9.067203e-01
 * time: 325.93768215179443
   283     1.011774e+03     9.565073e-01
 * time: 327.2270030975342
   284     1.011774e+03     9.628263e-01
 * time: 328.4484510421753
   285     1.011774e+03     9.654767e-01
 * time: 329.71515011787415
   286     1.011774e+03     9.654769e-01
 * time: 331.0440671443939
   287     1.011774e+03     9.656366e-01
 * time: 332.29603695869446
   288     1.011774e+03     9.656622e-01
 * time: 333.49823212623596
   289     1.011774e+03     9.656624e-01
 * time: 334.83915305137634
   290     1.011774e+03     9.656627e-01
 * time: 336.1911470890045
   291     1.011774e+03     9.660395e-01
 * time: 337.40075516700745
   292     1.011774e+03     9.661257e-01
 * time: 338.56860303878784
   293     1.011774e+03     9.663115e-01
 * time: 339.7686550617218
   294     1.011774e+03     9.662851e-01
 * time: 340.9534089565277
   295     1.011773e+03     9.654773e-01
 * time: 342.1841011047363
   296     1.011772e+03     9.623124e-01
 * time: 343.39579916000366
   297     1.011769e+03     9.521889e-01
 * time: 344.63091802597046
   298     1.011763e+03     9.239616e-01
 * time: 345.85337114334106
   299     1.011746e+03     9.242954e-01
 * time: 347.06980299949646
   300     1.011711e+03     1.257052e+00
 * time: 348.2797350883484
   301     1.011651e+03     1.361278e+00
 * time: 349.5131859779358
   302     1.011591e+03     9.076916e-01
 * time: 350.72918009757996
   303     1.011568e+03     3.084752e-01
 * time: 351.9666311740875
   304     1.011565e+03     2.946404e-01
 * time: 353.18393301963806
   305     1.011565e+03     2.821273e-01
 * time: 354.4147000312805
   306     1.011565e+03     2.766260e-01
 * time: 355.6049060821533
   307     1.011565e+03     2.778010e-01
 * time: 356.8090469837189
   308     1.011565e+03     2.830281e-01
 * time: 358.00295400619507
   309     1.011565e+03     2.842619e-01
 * time: 359.26140904426575
   310     1.011565e+03     2.861093e-01
 * time: 360.4436321258545
   311     1.011565e+03     2.861138e-01
 * time: 361.67281103134155
   312     1.011565e+03     2.867641e-01
 * time: 362.8731110095978
   313     1.011565e+03     2.867756e-01
 * time: 364.15695214271545
   314     1.011565e+03     2.884589e-01
 * time: 365.3516011238098
   315     1.011565e+03     2.897335e-01
 * time: 366.55631017684937
   316     1.011565e+03     2.927744e-01
 * time: 367.7528030872345
   317     1.011565e+03     2.967424e-01
 * time: 368.9650890827179
   318     1.011565e+03     3.027347e-01
 * time: 370.16223907470703
   319     1.011564e+03     3.099827e-01
 * time: 371.3749530315399
   320     1.011563e+03     3.155826e-01
 * time: 372.5697560310364
   321     1.011560e+03     3.082844e-01
 * time: 373.79016399383545
   322     1.011555e+03     2.625121e-01
 * time: 375.00247716903687
   323     1.011548e+03     2.136271e-01
 * time: 376.2607719898224
   324     1.011542e+03     1.165033e-01
 * time: 377.5011751651764
   325     1.011538e+03     3.914224e-02
 * time: 378.72695803642273
   326     1.011537e+03     3.083888e-02
 * time: 379.9249851703644
   327     1.011536e+03     2.766364e-02
 * time: 381.10187315940857
   328     1.011536e+03     1.873262e-02
 * time: 382.3302810192108
   329     1.011536e+03     5.453951e-03
 * time: 383.66944313049316
   330     1.011536e+03     4.882951e-03
 * time: 384.9326410293579
   331     1.011535e+03     4.331879e-03
 * time: 386.2310781478882
   332     1.011535e+03     2.711268e-03
 * time: 387.5538351535797
   333     1.011535e+03     4.474256e-04
 * time: 388.8774881362915
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: 7.104873657226562e-5
     1     4.804005e+05     8.280311e+05
 * time: 7.303060054779053
     2     3.595691e+05     6.035842e+05
 * time: 14.217710018157959
     3     1.691071e+05     2.702705e+05
 * time: 21.381021976470947
     4     9.193152e+04     1.451040e+05
 * time: 29.074352025985718
     5     4.768538e+04     6.630202e+04
 * time: 36.77451300621033
     6     2.904205e+04     3.416919e+04
 * time: 44.24269914627075
     7     1.840152e+04     1.671742e+04
 * time: 51.49464511871338
     8     1.280989e+04     1.022268e+04
 * time: 58.654086112976074
     9     9.713285e+03     9.298485e+03
 * time: 65.77444505691528
    10     7.628127e+03     8.330656e+03
 * time: 72.92816305160522
    11     6.236278e+03     7.340367e+03
 * time: 80.02186298370361
    12     5.271958e+03     6.303773e+03
 * time: 86.78265404701233
    13     4.411047e+03     4.994130e+03
 * time: 93.56431102752686
    14     3.732935e+03     3.538183e+03
 * time: 100.52936005592346
    15     3.399272e+03     2.450045e+03
 * time: 107.21587204933167
    16     3.302987e+03     1.848475e+03
 * time: 113.93394017219543
    17     3.291788e+03     1.642864e+03
 * time: 120.67175102233887
    18     3.290743e+03     1.594765e+03
 * time: 127.43211913108826
    19     3.289686e+03     1.562640e+03
 * time: 134.16877102851868
    20     3.286186e+03     1.496632e+03
 * time: 141.11416101455688
    21     3.277917e+03     1.402865e+03
 * time: 148.06146097183228
    22     3.255958e+03     1.254296e+03
 * time: 154.90790915489197
    23     3.200979e+03     1.038621e+03
 * time: 161.56987810134888
    24     3.063520e+03     7.403602e+02
 * time: 168.30342602729797
    25     2.730315e+03     3.768701e+02
 * time: 175.12595105171204
    26     1.934186e+03     2.246121e+02
 * time: 181.83757519721985
    27     1.704399e+03     2.219826e+02
 * time: 204.22278904914856
    28     1.471184e+03     2.025825e+02
 * time: 239.96606302261353
    29     1.316789e+03     1.554715e+02
 * time: 270.3780381679535
    30     1.226363e+03     1.775721e+02
 * time: 277.61546897888184
    31     1.206626e+03     2.781030e+02
 * time: 284.5865089893341
    32     1.197830e+03     2.430603e+02
 * time: 291.60668301582336
    33     1.190342e+03     2.327966e+02
 * time: 298.6062650680542
    34     1.178784e+03     2.105878e+02
 * time: 306.0143361091614
    35     1.177853e+03     2.228653e+02
 * time: 313.1887249946594
    36     1.177764e+03     2.256182e+02
 * time: 320.2303321361542
    37     1.177746e+03     2.259425e+02
 * time: 327.1361041069031
    38     1.177670e+03     2.264699e+02
 * time: 334.7166361808777
    39     1.177500e+03     2.264641e+02
 * time: 341.1346380710602
    40     1.177034e+03     2.244207e+02
 * time: 347.4957320690155
    41     1.175895e+03     2.160307e+02
 * time: 353.8993980884552
    42     1.173241e+03     1.906017e+02
 * time: 360.94166111946106
    43     1.168190e+03     1.332917e+02
 * time: 367.89776396751404
    44     1.161836e+03     5.291022e+01
 * time: 375.2413260936737
    45     1.158453e+03     6.595471e+01
 * time: 382.94063115119934
    46     1.157897e+03     6.411640e+01
 * time: 389.74897599220276
    47     1.157870e+03     6.098679e+01
 * time: 396.79437017440796
    48     1.157868e+03     6.032481e+01
 * time: 403.460196018219
    49     1.157864e+03     5.920991e+01
 * time: 410.1040461063385
    50     1.157856e+03     5.740417e+01
 * time: 416.6641221046448
    51     1.157832e+03     5.407031e+01
 * time: 423.2501881122589
    52     1.157774e+03     4.798188e+01
 * time: 429.78075098991394
    53     1.157628e+03     3.659183e+01
 * time: 436.37866497039795
    54     1.157299e+03     3.707862e+01
 * time: 443.1555600166321
    55     1.156690e+03     3.453183e+01
 * time: 450.6136510372162
    56     1.155959e+03     4.268473e+01
 * time: 457.9204480648041
    57     1.155557e+03     5.000309e+01
 * time: 464.72136306762695
    58     1.155472e+03     4.219411e+01
 * time: 471.3585751056671
    59     1.155463e+03     3.894982e+01
 * time: 478.0008611679077
    60     1.155462e+03     3.872028e+01
 * time: 484.5322630405426
    61     1.155458e+03     3.823435e+01
 * time: 491.1562731266022
    62     1.155450e+03     3.747328e+01
 * time: 497.70220708847046
    63     1.155426e+03     3.608641e+01
 * time: 504.95913219451904
    64     1.155365e+03     3.361168e+01
 * time: 512.3541111946106
    65     1.155207e+03     2.894844e+01
 * time: 519.2790191173553
    66     1.154814e+03     2.504332e+01
 * time: 526.0067319869995
    67     1.153910e+03     4.539953e+01
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    68     1.152231e+03     7.425240e+01
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    69     1.150287e+03     7.473669e+01
 * time: 546.1285421848297
    70     1.149189e+03     5.606074e+01
 * time: 552.926705121994
    71     1.148944e+03     4.801310e+01
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    72     1.148932e+03     4.659470e+01
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    73     1.148930e+03     4.635167e+01
 * time: 573.6650211811066
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    75     1.148907e+03     4.466140e+01
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    76     1.148865e+03     4.249800e+01
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    78     1.148491e+03     3.552369e+01
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    80     1.146737e+03     4.466673e+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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    84     1.144341e+03     6.591571e+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
 * time: 666.7742261886597
    88     1.144281e+03     5.369030e+01
 * time: 673.2548730373383
    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
 * time: 692.662024974823
    92     1.142498e+03     3.276758e+01
 * time: 699.1454911231995
    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
 * time: 718.7622311115265
    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
 * time: 763.4485530853271
   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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   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
 * time: 828.6337280273438
   113     1.136674e+03     3.790005e+01
 * time: 835.3996031284332
   114     1.136604e+03     3.705604e+01
 * time: 841.7789499759674
   115     1.136418e+03     3.550032e+01
 * time: 848.1886880397797
   116     1.135940e+03     3.257323e+01
 * time: 854.5779731273651
   117     1.134744e+03     2.697619e+01
 * time: 861.1727201938629
   118     1.132016e+03     4.861352e+01
 * time: 868.3113050460815
   119     1.127206e+03     6.900228e+01
 * time: 875.400288105011
   120     1.122365e+03     5.795238e+01
 * time: 882.0887541770935
   121     1.119760e+03     2.908163e+01
 * time: 888.8622992038727
   122     1.118972e+03     2.137897e+01
 * time: 895.5849189758301
   123     1.118870e+03     1.909827e+01
 * time: 902.4367489814758
   124     1.118863e+03     1.886936e+01
 * time: 909.1608180999756
   125     1.118862e+03     1.852746e+01
 * time: 916.236958026886
   126     1.118861e+03     1.864804e+01
 * time: 923.0221049785614
   127     1.118861e+03     1.862831e+01
 * time: 929.771320104599
   128     1.118860e+03     1.865947e+01
 * time: 936.0619740486145
   129     1.118859e+03     1.869546e+01
 * time: 942.5354809761047
   130     1.118856e+03     1.878621e+01
 * time: 948.935455083847
   131     1.118850e+03     1.894171e+01
 * time: 955.261815071106
   132     1.118834e+03     1.922062e+01
 * time: 961.7090041637421
   133     1.118794e+03     1.967952e+01
 * time: 968.2986810207367
   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
 * time: 994.6035871505737
   138     1.111680e+03     7.743613e+01
 * time: 1001.3775629997253
   139     1.102031e+03     7.499119e+01
 * time: 1008.4734649658203
   140     1.090653e+03     6.250170e+01
 * time: 1015.5609891414642
   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
 * time: 1067.5198121070862
   148     1.084973e+03     2.065532e+01
 * time: 1074.803817987442
   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: 1132.6221840381622
   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
 * time: 1155.7297821044922
   160     1.084701e+03     2.318755e+01
 * time: 1162.957458972931
   161     1.084458e+03     2.380805e+01
 * time: 1170.0839080810547
   162     1.083881e+03     2.403001e+01
 * time: 1177.5141251087189
   163     1.082583e+03     2.865879e+01
 * time: 1184.8182201385498
   164     1.081739e+03     3.341843e+01
 * time: 1192.5942010879517
   165     1.078499e+03     5.054188e+01
 * time: 1200.6273641586304
   166     1.075168e+03     6.966535e+01
 * time: 1207.7848591804504
   167     1.069624e+03     6.533378e+01
 * time: 1214.9108669757843
   168     1.068098e+03     3.856272e+01
 * time: 1221.9561531543732
   169     1.065571e+03     1.611062e+01
 * time: 1229.0501780509949
   170     1.065230e+03     1.527933e+01
 * time: 1236.1949791908264
   171     1.065088e+03     1.385503e+01
 * time: 1245.0359981060028
   172     1.065086e+03     1.382742e+01
 * time: 1252.3673050403595
   173     1.065086e+03     1.383663e+01
 * time: 1259.2403700351715
   174     1.065086e+03     1.384273e+01
 * time: 1266.1264770030975
   175     1.065086e+03     1.385109e+01
 * time: 1272.9880511760712
   176     1.065084e+03     1.386455e+01
 * time: 1279.9624600410461
   177     1.065082e+03     1.388591e+01
 * time: 1286.9271290302277
   178     1.065074e+03     1.391915e+01
 * time: 1293.8261210918427
   179     1.065053e+03     1.397116e+01
 * time: 1300.8573679924011
   180     1.065000e+03     1.742031e+01
 * time: 1308.2113420963287
   181     1.064862e+03     2.855619e+01
 * time: 1315.3242671489716
   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
 * time: 1343.2502610683441
   186     1.052919e+03     1.304962e+02
 * time: 1350.1511101722717
   187     1.048870e+03     1.445092e+02
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   188     1.037225e+03     8.004893e+01
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   189     1.035989e+03     6.689301e+01
 * time: 1372.1560151576996
   190     1.035708e+03     3.351073e+01
 * time: 1379.3331661224365
   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: 1460.313749074936
   202     1.035046e+03     3.158994e+00
 * time: 1467.4917101860046
   203     1.035046e+03     3.157781e+00
 * time: 1474.9544219970703
   204     1.035046e+03     3.155534e+00
 * time: 1481.9223191738129
   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
 * time: 1503.6791470050812
   208     1.035033e+03     3.104858e+00
 * time: 1510.7573671340942
   209     1.035012e+03     3.049096e+00
 * time: 1517.54714012146
   210     1.034957e+03     3.798101e+00
 * time: 1524.3876831531525
   211     1.034816e+03     6.029607e+00
 * time: 1531.3207931518555
   212     1.034472e+03     9.154140e+00
 * time: 1538.2868700027466
   213     1.033706e+03     1.258550e+01
 * time: 1545.1632590293884
   214     1.032296e+03     1.427962e+01
 * time: 1552.0286631584167
   215     1.030400e+03     1.353250e+01
 * time: 1559.3697700500488
   216     1.029562e+03     1.451097e+01
 * time: 1567.1186401844025
   217     1.028793e+03     1.534582e+01
 * time: 1574.1011061668396
   218     1.028722e+03     2.309928e+01
 * time: 1581.254147052765
   219     1.028409e+03     1.971948e+01
 * time: 1588.248957157135
   220     1.028326e+03     1.936501e+01
 * time: 1595.1934130191803
   221     1.028234e+03     2.040540e+01
 * time: 1602.3653500080109
   222     1.028219e+03     2.201826e+01
 * time: 1609.3723981380463
   223     1.028180e+03     2.177994e+01
 * time: 1617.0668680667877
   224     1.028176e+03     2.186278e+01
 * time: 1625.5273001194
   225     1.028175e+03     2.201733e+01
 * time: 1633.2632901668549
   226     1.028174e+03     2.197587e+01
 * time: 1640.3835501670837
   227     1.028174e+03     2.195893e+01
 * time: 1647.5778830051422
   228     1.028172e+03     2.186809e+01
 * time: 1654.7768139839172
   229     1.028169e+03     2.178132e+01
 * time: 1661.9195091724396
   230     1.028160e+03     2.162024e+01
 * time: 1669.1522631645203
   231     1.028139e+03     2.139041e+01
 * time: 1676.2342910766602
   232     1.028082e+03     2.100359e+01
 * time: 1683.7002580165863
   233     1.027936e+03     2.808708e+01
 * time: 1691.2276241779327
   234     1.027561e+03     4.544763e+01
 * time: 1698.450073003769
   235     1.026628e+03     7.187426e+01
 * time: 1705.7756099700928
   236     1.024510e+03     1.065337e+02
 * time: 1712.837678194046
   237     1.020621e+03     1.329824e+02
 * time: 1720.2242259979248
   238     1.015709e+03     1.045562e+02
 * time: 1727.6427659988403
   239     1.014365e+03     2.918128e+01
 * time: 1734.700317144394
   240     1.013219e+03     1.803992e+01
 * time: 1742.156051158905
   241     1.012886e+03     1.450533e+01
 * time: 1749.6614170074463
   242     1.012748e+03     9.039639e+00
 * time: 1756.8300030231476
   243     1.012704e+03     2.255849e+00
 * time: 1764.004240989685
   244     1.012698e+03     1.107778e+00
 * time: 1771.012379169464
   245     1.012697e+03     9.968755e-01
 * time: 1777.8426251411438
   246     1.012697e+03     9.970599e-01
 * time: 1784.708956003189
   247     1.012697e+03     9.971842e-01
 * time: 1791.584792137146
   248     1.012697e+03     9.972169e-01
 * time: 1798.8590910434723
   249     1.012697e+03     9.972169e-01
 * time: 1806.9139811992645
   250     1.012697e+03     9.973243e-01
 * time: 1814.312350988388
   251     1.012697e+03     9.973078e-01
 * time: 1822.1184921264648
   252     1.012697e+03     9.972048e-01
 * time: 1830.1026060581207
   253     1.012697e+03     9.970536e-01
 * time: 1836.8492109775543
   254     1.012697e+03     9.967402e-01
 * time: 1843.6247491836548
   255     1.012697e+03     9.961565e-01
 * time: 1850.383197069168
   256     1.012696e+03     9.949478e-01
 * time: 1857.0548510551453
   257     1.012694e+03     1.312115e+00
 * time: 1864.1534781455994
   258     1.012690e+03     2.118030e+00
 * time: 1871.3820061683655
   259     1.012679e+03     3.352490e+00
 * time: 1878.3617889881134
   260     1.012651e+03     5.076526e+00
 * time: 1885.2500751018524
   261     1.012594e+03     6.891495e+00
 * time: 1892.237053155899
   262     1.012501e+03     7.239074e+00
 * time: 1899.1164109706879
   263     1.012418e+03     4.479611e+00
 * time: 1905.7245399951935
   264     1.012390e+03     1.267149e+00
 * time: 1912.3669211864471
   265     1.012386e+03     8.840304e-01
 * time: 1918.8868100643158
   266     1.012386e+03     8.549097e-01
 * time: 1925.4801540374756
   267     1.012386e+03     8.516869e-01
 * time: 1932.0459461212158
   268     1.012386e+03     8.516869e-01
 * time: 1939.9808990955353
   269     1.012386e+03     8.516869e-01
 * time: 1948.0971901416779
   270     1.012386e+03     8.516817e-01
 * time: 1955.1287310123444
   271     1.012386e+03     8.516817e-01
 * time: 1963.1566500663757
   272     1.012386e+03     8.516817e-01
 * time: 1971.608798980713
   273     1.012386e+03     8.516817e-01
 * time: 1979.764573097229
   274     1.012386e+03     8.516817e-01
 * time: 1988.4256429672241
   275     1.012386e+03     8.516817e-01
 * time: 1997.481558084488
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: 7.390975952148438e-5
     1     4.804005e+05     8.280312e+05
 * time: 1.1732280254364014
     2     3.595691e+05     6.035843e+05
 * time: 2.242790937423706
     3     1.691071e+05     2.702705e+05
 * time: 3.2214529514312744
     4     9.193152e+04     1.451040e+05
 * time: 4.302997827529907
     5     4.768539e+04     6.630202e+04
 * time: 5.33185887336731
     6     2.904205e+04     3.416919e+04
 * time: 6.380807876586914
     7     1.840153e+04     1.671742e+04
 * time: 7.409325838088989
     8     1.280989e+04     1.022268e+04
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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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   266     1.012386e+03     8.511780e-01
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   267     1.012386e+03     8.489977e-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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   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: 336.07075095176697
   317     1.011565e+03     2.967424e-01
 * time: 337.10646986961365
   318     1.011565e+03     3.027347e-01
 * time: 338.1563129425049
   319     1.011564e+03     3.099827e-01
 * time: 339.23011898994446
   320     1.011563e+03     3.155826e-01
 * time: 340.27930903434753
   321     1.011560e+03     3.082844e-01
 * time: 341.36858201026917
   322     1.011555e+03     2.625121e-01
 * time: 342.4211208820343
   323     1.011548e+03     2.136271e-01
 * time: 343.5135009288788
   324     1.011542e+03     1.165033e-01
 * time: 344.5726819038391
   325     1.011538e+03     3.914224e-02
 * time: 345.66385984420776
   326     1.011537e+03     3.083888e-02
 * time: 346.6986348628998
   327     1.011536e+03     2.766364e-02
 * time: 347.75840497016907
   328     1.011536e+03     1.873262e-02
 * time: 348.8256039619446
   329     1.011536e+03     5.453951e-03
 * time: 349.87309885025024
   330     1.011536e+03     4.882951e-03
 * time: 350.9379668235779
   331     1.011535e+03     4.331879e-03
 * time: 352.02713799476624
   332     1.011535e+03     2.711268e-03
 * time: 353.317409992218
   333     1.011535e+03     4.474256e-04
 * time: 354.52402782440186
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.