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
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.