WKB approximation for the reaction-path Hamiltonian: Application to variational transition state theory, vibrationally adiabatic excited-state barrier heights, and resonance calculations

We present several kinds of calculations, for reaction rates, vibrationally adiabatic barrier heights, and resonance energies and widths, in which we use the WKB approximation for vibrational energies of stretches in the reaction-path Hamiltonian. We consider both collinear and three-dimensional atom-transfer reactions. As compared to previous calculations employing the Morse approximation for vibrational energies of stretches, there is generally significant quantitative improvement in accuracy for ground-state quantities and thermal reaction rates, and there is dramatic improvement in accuracy for excited-state quantities. We also update our predictions for some three-dimensional reaction rates calculated with an accurate ab initio potential energy surface.