Considerations in Geometry Optimization

The default settings in MOPAC are designed to allow most systems to be optimized in an efficient way. Quite often, however, problems arise. The following notes are intended as background material for use when things go wrong.

Overriding the default options

In the EigenFollowing geometry optimization method, the geometry is changed on each cycle; if the ΔHf  decreases, the cycle is completed. If it does not drop, the step-size is reduced, and the ΔHf  recalculated. Only when the ΔHf  decreases, compared to the previous cycle, is the current cycle considered to be successful. During the calculation, the confidence level or trust radius is continuously checked. If this becomes too small, the calculation will be stopped. This can readily happen if (a) the geometry was already almost optimized; (b) a reaction path or grid calculation is being performed; (c) if the geometry is in internal coordinates and "big rings" are involved; or (d) if the gradients are not correctly calculated (in a complicated C.I., for example).

For cases (a) and (b), add LET and DDMIN=0. In case (c) use either mixed coordinates or entirely Cartesian coordinates. Case (d) is difficult--if nothing else works, add NOANCI; this will always cause the derivatives to be correctly calculated, but will also use a lot of time.

Adding LET and DDMIN=0 is often very effective, particularly when reaction paths are being calculated. The first geometry optimization might take more cycles, but the resulting Hessian matrix is better tempered, and subsequent steps are generally more efficient.