“Direct” and “Correct” Calculation of Canonical and Microcanonical Rate Constants for Chemical Reactions

Theoretical approaches for calculating rate constants of chemical reactionsseither the microcanonical rate for a given total energy k(E) or the canonical rate for a given temperature k(T)sare described that are both “direct”, i.e., bypass the necessity of having to solve the complete state-to-state quantum reactive scattering problem, yet also “correct”, i.e., in principle exact (given a potential energy surface, assuming nonrelativistic quantum mechanics, etc.) Applications to a variety of reactions are presented to illustrate the methodology for various dynamical situations, e.g., transition-state-theory-like dynamics where the system moves directly through the interaction (transition-state) region and reactions that form long-lived collision complexes. It is also shown how this rigorous quantum theory can be combined with the Lindemann mechanism for describing the effects of collisions with a bath gas, so as to be able to treat recombination reactions and other effects of pressure. Finally, several ways are discussed for combining these rigorous approaches for small molecule dynamics with an approximate treatment of (perhaps many) other degrees of freedom (i.e., a solvent, a substrate, a cluster environment) that may be coupled to it.