The Representation and use of Potential Energy Surfaces in the Wide Vicinity of a Reaction Path for Dynamics Calculations on Polyatomic Reactions

Abstract

We consider three aspects of potential energy surface representations for dynamics calculations on polyatomic systems, with special emphasis on generalized transition state theory and tunneling calculations, (i) We present methods for calculating the vibrational energies of generalized transition states from either a cartesian or internal coordinate force field and including the effect of raode-mode coup!.ings on the rate constant by perturbation theory and the Pitzer-Gwinn approximation, (ii) We discuss practical aspects in the use of ab initio gradient-based electronic structure calculations for the calculation of cartesian force fields for a set of stationary points on the potential energy surface or for a sequence of generalized transition states. (iii) We discuss recent progress on the development of global analytic representations for potential energy surfaces of polyatomic reactions. Such global representations can be used to generate either cartesian or internal-coordinate force fields for generalized transition states, and they can also be used to compute the potential energy surface far from the minimum energy path as may be required for tunneling calculations in some cases.