Efficient Molecular Mechanics for Chemical Reactions:  Multiconfiguration Molecular Mechanics Using Partial Electronic Structure Hessians

Multiconfiguration molecular mechanics (MCMM) is a method for representing polyatomic potential energy surfaces by combining molecular mechanics potentials for the reactant and product wells with electronic structure data (energy, gradient, and Hessian) at the saddle point and a small number of nonstationary points. A general strategy for placement of the nonstationary points has been developed [Albu, T. V.; Corchado, J. C.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 8465] for fitting potential energy surfaces in the vicinity of the reaction path and in the reaction swath region and for calculating rate constants for atom transfer reactions by variational transition state theory with multidimensional tunneling. In the present work, we improve the efficiency of the MCMM method by using electronic structure calculations only for certain critical elements of the Hessians at the nonstationary points and by using interpolation for the other elements at the nonstationary points. We tested this new MCMM strategy for a diverse test suite of six reactions involving hydrogen-atom transfer. The new method yields quite accurate rate constants as compared with the standard MCMM strategy employing full electronic structure Hessians and also as compared with direct dynamics calculations using an uninterpolated full potential energy surface at the same electronic structure level. In comparison with the standard MCMM strategy, this new procedure reduces the computational effort associated with the nonstationary points by a factor of up to 3 for the test reactions and up to 11 for even larger reactive systems.