The quantum transition state wavepacket method

Abstract

The accurate calculation of thermal rate constants for reactions in the gas phase often requires both accurate potential energy surfaces (PESs) and the use of quantum mechanics, particularly in the case of light atom (H) transfers in reactions with activation energy barriers between reactants and products. The thermal rate constant k(T) can be calculated directly or as a thermal average over the cumulative reaction probability N(E). Both k(T) and N(E) can be calculated exactly and directly in terms of flux formulations first presented by Miller etal. In this paper we review the recent reformulation of the calculation of N(E) in terms of the time evolution of transition state wavepackets (TSWPs), which then provides a very effective method for reactions with activation energy barriers. This method requires a single time propagation for each TSWP contributing to the desired thermal rate constant from which the required contributions to N(E) for all E can be obtained. We then apply this to the calculation of N(E) and k(T) for the interesting four atom reaction H₂(D₂)+CN→HCN(DCN)+H(D). The system has a metastable well in the PES at the linear CNHH configuration. The results and a discussion of the influence of this secondary TS well are presented.