Potential Energy Characteristics for Chemical Reactions

Abstract

Ab initio molecular orbital (MO) methods can provide accurate descriptions of potential energy surfaces for chemical reactions. For triatomic systems such as H3 and H2F extensive calculations of very accurate potential surfaces have been carried out. For systems having four atoms or more, however, the accomplishment so far has been rather limited. One of the reasons for this is that the full potential energy surface is a function of 3N-6 coordinates where N is the number of atoms, and this is too many degrees of freedom for the full surface to be mapped. Even to optimize the geometrical parameters for all the degrees of freedom to determine the reactant and saddle point geometries is very difficult. However, a revolution is taking place in the way quantum chemists probe a potential surface. It is based on the development of new theoretical methods and computer programs for analytically calculating the gradient of the energy with respect to the nuclear coordinates.1 Quantum chemists are now learning how to use this energy gradient, a concept more familiar in the context of collision theory, to study the characteristics of potential energy surfaces. In this review we will first summarize briefly how to calculate the energy gradient analytically. Then we will discuss methods of determining geometries, force constants, and normal vibrations for both equilibrium configurations and saddle points, and we will show several examples of such calculations we have recently carried out.