The root finder used by the ODE solver of the CIF simulator, can be configured using several options.
The ODE solver root finder maximum check interval option (ODE solver: ODE root finder category) is explained on a separate page. See Problems with root finding.
The ODE solver root finding algorithm option (ODE solver: ODE root finder category) can be used to configure the root finding algorithm to use. The following algorithms are available:
Regula Falsi (False position) method
Pegasus method (default)
The Illinois and Pegasus methods are modified Regula Falsi methods. The algorithms differ only in how they choose the middle point of the interval. This influences their convergence speed. The Regula Falsi method in particular should be avoided in practice.
The ODE solver root finder absolute tolerance option (ODE solver: ODE root finder category) and the ODE solver root finder relative tolerance option (ODE solver: ODE root finder category), can be used to configure how precise the results of the root finder should be. The lower the tolerance (or error), the higher the precision of the results, but also the more computing time it costs to get to that precision. The absolute precision is the difference between the prediction and the actual value, while the relative precision is that same difference, as a fraction of the actual value. In the latter case, precision reduces with increasing values.
The ODE solver root finder maximum iterations option (ODE solver: ODE root finder category) can be used to set the maximum number of iterations to use for root finding. This value must be at least one. If the root interval is larger than the tolerances allow, after the maximum number of iterations, root finding fails.
If it fails for your CIF specification, increase the value of this option. If that does not help, your specification is most likely too complex (by nature), or has a modeling error that causes the specification to become too complex for the root finding algorithm to handle. The problem however, may also be in one of the other root finder options, or in one of the integrator options.