Quantum mechanical computational studies of chemical reactions: I. Close-coupling method for the collinear H + H₂reaction

A fully converged, close-coupling calculation, using natural collision coordinates, is presented for the collinear H + H2 system, on a Porter-Karplus potential energy surface in the energy range of 9 to 35 kcal/mole. Eight closed channels were required for convergence in the threshold region. Inclusion of the closed channels alters considerably the numerical results. The effects of the vibrational non-adiabatic coupling terms were demonstrated. Particularly pronounced is the influence of the (energy dependent) inertial centrifugal energy due to the curvature of the reaction path. No evidence for tunnelling was obtained. The considerable difference between the quantal and classical reaction probabilities in the threshold energy region is entirely due to the non-adiabatic coupling terms, which tend to reduce the kinetic energy available along the reaction path, more so in the classical case, thereby leading to a higher dynamic threshold. The Arrhenius plot of the collinear rate constant shows more curvature in the quantal case due to the different threshold behaviour caused by the vibrationally non-adiabatic coupling.