Recommendations To Geometry Optimizations
It is recommened to perform geometry optimizations using analytically evaluated gradients, as optimizations based on numerical gradients can be rather costly.
Furthermore, it is recommended to use (a) a good starting geometry preferably optimized at a lower computational level, as in this way the number of cycles can be reduced and (b) to use a as good as possible force-constant matrix. The latter point also helps to ensure fast convergence.
If geometry optimizations are carried out for a subsequent calculation of anharmonic force fields or a vibrational averaging of properties, it is strongly recommended to use tighter thresholds in the optimization as well as in the corresponding quantum-chemical calculation.