Wednesday, March 30, 2016

Development of a True Transition State Force Field from Quantum Mechanical Calculations

Ádám Madarász, Dénes Berta, and Robert S. Paton (2016)
Contributed by Jan Jensen

A true TS FF (TTSFF) is a force field for which the TS is a true saddle point on the PES, i.e. an unconstrained geometry with a zero gradient and exactly one imaginary frequency.  The idea is that you develop the FF parameters for each TS of interest from QM calculations and then use the TTSFF to do a conformational search to find the lowest energy TS structure. The TTSFF itself cannot be used to compute the barrier.

The main trick to making this work is to include harmonic stretch terms between 1-3 atom pairs (i.e. Urey-Bradley) terms in the FF expression. Some trial-and-error is required in selecting which FF parameters to optimise and exactly what to optimise against.

The approach is developed primarily to study selectivity, where the relative energies of TS structures determine which product is made.  However, one could imagine several other potential uses.  For example, one could probably parameterise the TTSFF using a small structural model and then use the TTSFF to find TS structures with large substituents, which would then be used a initial guesses for a QM TS search.  Furthermore, if QM//(TTS)FF barriers turn out to be reasonably accurate then one could save a tremendous amount of CPU time.

I thank @CompChemNews for bringing this paper to my attention

This work is licensed under a Creative Commons Attribution 4.0