Gluing potential energy surfaces with rare event simulations.

We develop a new method combining replica exchange transition interface sampling with two distinct potential energy surfaces. The method can be used to combine different levels of theory in a simulation of a molecular process (e.g., a chemical reaction), and it can serve as a dynamical version of QM-MM, connecting classical dynamics with Ab Initio dynamics in the time domain. This new method, which we coin QuanTIS, could be applied to use accurate but expensive density functional theory based molecular dynamics for the breaking and making of chemical bonds, while the diffusion of reactants in the solvent are treated with classical force fields. We exemplify the method by applying it to two simple model systems (an ion dissociation reaction and a classical hydrogen model), and we discuss a possible extension of the method in which classical force field parameters for chemical reactions can be optimized on the fly.