Ab initio calculation of proton-hydrocarbon scattering cross sections

The prediction of scattering cross sections for atom-molecule collisions by means of ab initio electronic structure methods is discussed with reference to recent calculations for the proton-methane and proton-ethyne systems, potential energy surfaces and nonadiabatic coupling elements are computed employing the multireference single- and double-excitation configuration interaction (MRD-CI) method. These data are then taken as input for either a semiclassical or a fully quantum treatment to compute scattering cross sections for both elastic and inelastic processes. The role of molecular symmetry in determining both the shapes of the potential curves and the radial coupling elements between different channels is discussed and illustrated with numerous examples. Nonadiabatic couplings cause transitions between molecular states and their effect on the computed differential and total cross sections for charge transfer and elastic processes for different proton approaches are compared for the two collision systems.