2C: The MOPAC Saddle algorithm

This method, invoked by the keyword SADDLE, calculates a kind of 'average' between two structures given in the input file. Iteratively, it will pull the lower energy structure a small step in the direction (determined by the difference vector) of the other one.
It will end up in a transition state if two geometries, one on each side of the hill top, are supplied, that are related by a continuous deformation.
It can be tried in the case of ring deformations, where it is difficult to select one or two torsions which are 'responsible' for the conformational change. By looking at the saddle output one can get a hint which torsions describe the change best.
Saddle can be run using z-matrices, in which case special care should be taken that the dihedral angles are connected in a continuous way.
If this is a problem, it can be made to run in cartesian coordinates using the keyword XYZ.

Back to chapter 2, How to locate a Transition State.