ODE error messages

When the ODE solver returns an error code, Phoenix NLME reports the error messages to the user so that appropriate actions may be taken. The error messages may appear in either Core Status text output or Warnings and Errors text output or both. If any ODE error message appears in the Warnings and Errors text output, then the corresponding results obtained, if there are, should not be trusted and/or be interpreted with care.

For the estimation mode, the ODE error message may appear in either Core Status text output or both Core Status text output and Warnings and Errors text output. If there are occasional occurrences of ODE error messages during some early or intermediate iterations, then it is probably due to some unrealistic parameter values found during the intermediate search, and hence there is typically no need to worry about these (as the optimization is eventually able to go to the right direction). However, if the error message appears during almost all the iterations, then the engine may stuck in a bad/inappropriate region, and hence the estimation results obtained may not be reliable. Moreover, if the error message continues to show up during the standard error calculations step and/or during the final stage for preparing worksheet outputs (e.g., Residual and Overall worksheets), then the corresponding results obtained should not be trusted.

For the simulation modes (including VPC), a 0-iteration fit is performed before the simulation run to populate worksheet outputs. Errors that occur during the 0-iteration fit or simulation runs are recorded in files and reported in the Warnings and Errors text output and the corresponding results obtained, if there are any, should not be trusted. For example, if ODE errors occurred during generation of simulation table worksheets, then simulation tables should be taken with care; if ODE errors occurred during predictive check step, predcheck outputs should be taken with care. It is worth pointing out that 0-iteration fit error files are wiped by subsequent simulation runs. To see them, the user should re-run the model in simple mode.

Typically, the ODE errors can be avoided by taking appropriate actions. For example, if the error message is about maximum number of function evaluations exceeded, then one can increase the value of “ODE max step” (by clicking the “Advanced <<” button in the Run Option tab) to be sufficiently large to avoid such error. While, for some cases, the easiest/best way is just to switch to a different solver. For example, if the model is suspected to be stiff, then one needs to switch to auto-detect or stiff solver (from the “max ODE” menu located in the Run Options tab).