Improving the Q2MM method for transition state force field modeling.

The Quantum-to-molecular mechanics method (Q2MM) for converting quantum mechanical transition states (TSs) to molecular mechanical minima has been modified to allow a fit to the "natural" reaction mode eigenvalue. The resulting force field gives an improved representation of the energy curvature at the TS, but can potentially give false responses to steric interactions. Ways to address this problem while staying close to the "natural" TS force field are discussed.