Molecular mechanics for chemical reactions: A standard strategy for using multiconfiguration molecular mechanics for variational transition state theory with optimized multidimensional tunneling

Multiconfiguration molecular mechanics (MCMM) is an extension of molecular mechanics to chemically reactive systems. This dual-level method combines molecular mechanics potentials for the reactant and product configurations with electronic structure Hessians at the saddle point and a small number of nonstationary points to model the potential energy surface in the reaction swath region between reactants and products where neither molecular mechanics potential is valid. The resulting semiglobal potential energy surface is used as input for dynamics calculations of tunneling probabilities and variational transition state theory rate constants. In this paper, we present a standard strategy for applying MCMM to calculate rate constants for atom transfer reactions. In particular, we propose a general procedure for determining where to calculate the electronic structure Hessians. We tested this strategy for a diverse test suite of six reactions involving hydrogen-atom transfer. It yields reasonably accurate rate constants as compared to direct dynamics using an uninterpolated full potential energy surface at the same electronic structure level. Furthermore, the rate constants at each of several successively more demanding levels of dynamical theory are also predicted accurately, which indicates that the MCMM potential energy surface accurately predicts many different details of the potential energy surface with a limited number of electronic structure Hessians.