Classical versus quantum mechanical calculations of the dynamics of elementary reactions: From state-resolved cross sections to rate constants

The performance of the quasiclassical trajectory method for the study of elementary chemical reactions is reviewed. Classical calculations of dynamical observables from state resolved differential cross sections to thermal rate constants for the most usual prototypic atom-diatom reactions (H + H-2, F + H-2, Cl + H-2, O(D-1) + H-2), for which reliable potential energy surfaces are available, are analyzed in the light of the most recent quantum mechanical results and experimental findings. In general, quasiclassical trajectory reaction cross sections and rate constants are in good agreement with their quantum mechanical and experimental counterparts, indicating that the motion of the nuclei during reactive encounters is largely classical for the systems considered, and that quantum effects, such as tunneling, play a relatively minor role in the overall dynamics. The importance of the zero-point energy of the transition state is highlighted as one of the most important deficiencies of the quasiclassical treatment. Finally, the need of a precise quasiclassical and quantal simulation of the actual laboratory measurements in order to clearly identify experimental quantum effects is emphasized.