Some models contain very rapid changes in the species concentrations or other variables being integrated by the solver. The solver will therefore need to take very small timesteps to capture these rapid changes, which results in a longer run time.

Typical examples include fast reactions (e.g. pH adjustment / neutralization, reactions with ionic equilibria); fast mass transfer (high kLa to ensure equilibrium between phases), models where phases appear or disappear. The variable causing the small timestep can be identified by viewing the Event Log in Simulator. This should give an indication of the source of the problem.

To correct this try modifying the model (either reduce very large parameter values or simplify the model). If this cannot be done, then some other options are available to reduce run times:

- Try another Simulator solver /
**integration method**. Occasionally the default solver (Rosenbrock) is slower or less robust than one of the other solvers. Try running in simulator with a different solver. If significant speed-ups are obtained by changing solver, this solver can be selected in the fitting exercise using the advanced options window. - Try reducing Simulator solver
**accuracy**in the**Advanced Options**window (click to open). As a rule, the higher the accuracy, the longer the solver run time will be. It may be possible to reduce the solver accuracy without affecting the solver results significantly. This will normally have only a small effect on the run time, but in some cases can be significant.