Ab initio dynamics: HeH+ + H2 → He + H3+ (C2/subv/) classical trajectories using a quantum mechanical potential-energy surface

Tabular values of near-Hartree-Fock ab initio energies for the ground electronic state of HeH3+ were expressed in analytical form by fitting with spline interpolation functions. The interpolation functions were then used to provide the potential-energy gradients necessary for dynamical calculations. General features of this method for representing potential-energy surfaces are discussed. Classical trajectories have been computed for the ion-molecule reaction HeH++H2 → He+H3++2.6 eV, constrained to follow C2ν symmetry reaction paths for which there is no energy barrier. Trajectories were run under the conditions: relative translational energies 0.001 and 0.1298 eV and internal vibrational states of reactants with quantum numbers 0 and 2; the initial vibrational phases of each trajectory were selected randomly. Analysis of the results revealed that most of the energy available to the products (including the exothermicity of the reaction) ends up as H3+ vibrational energy; more than half of the collisions produced greater than 90% vibrational energy conversion independent of temperature but decreasing slightly for higher initial vibrational states.