Ab initio potential energy surface for the ground (2A′) state of H+SiO and rotationally inelastic collision cross sections for circumstellar H+SiO collisions

Abstract

An ab initio potential energy surface has been calculated for the ground electronic state of the H+SiO system. The calculations were performed by first characterizing all the critical points of the system using ab initio complete active space-self consistent field (CAS-SCF) methods. These preliminary calculations were followed by multireference configuration interaction (CI) calculations covering a grid of 1748 nuclear geometries. The surface has been fitted to a global analytic form, using the recently proposed Reproducing Kernel Hilbert Space Method of Ho et al., [J. Chem. Phys. 104, 2584 (1996)] and preliminary vibrationally-rotationally inelastic scattering cross sections have been computed using infinite order sudden quantum scattering methods. The motivation of the research is the evaluation of these inelastic cross sections which are needed in the modeling of circumstellar SiO maser radiation. Significant structure has been observed in the computed inelastic cross sections. This structure is of the form which might be expected to give rise to the vibrational–rotational quantum state population inversions which in turn leads to the observed circumstellar maser action. An interesting feature of the computed potential energy surface is a line of maxima in nonlinear geometries. These maxima appear to arise from a nonsymmetry related conical intersection.