NONADIABATIC TRANSITION-STATE THEORY AND MULTIPLE POTENTIAL-ENERGY SURFACE MOLECULAR-DYNAMICS OF INFREQUENT EVENTS

Classical transition state theory (TST) provides the rigorous basis fo r the application of molecular dynamics (MD)to infrequent ever;ts, i.e ., reactions that are slow due to a high energy barrier. The TST rate is simply the equilibrium flux through a surface that divides reactant s from products. In order to apply MD to infrequent events, correction s to the TST late that account for recrossings of the dividing surface -are computed by starting trajectories at die dividing surface and int egrating them backward and forward in time. Both classical TST and con ventional MD invoke the adiabatic approximation, i.e., the, assumption that nuclear motion evolves on a single potential energy surface. Man y chemical rate processes involve multiple potential energy surfaces, however, and a number of ''surface-hopping'' MD methods have been deve loped in order to incorporate nonadiabatic transitions among the poten tial energy surfaces. In this paper we generalize TST to processes inv olving multiple potential energy surfaces. This provides the framework for a new method for MD simulation of infrequent events for reactions that evolve on multiple potential energy surfaces. We show how this m ethod can be applied rigorously even in conjunction; with phase-cohere nt surface-hopping methods, where the probability of switching potenti al energy surfaces-depends on the history of the trajectory, so integr ating trajectories backward to calculate the recrossing correction is problematic. We illustrate this new method by applying it in conjuncti on with the ''molecular dynamics with quantum transitions'' (MDQT) sur face-hopping method to a one-dimensional two-state barrier crossing pr oblem. (C) 1995 American Institute of Physics.