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

An ab initio potential energy surface has been calculated for the ground el ectronic state of the H+SiO system. The calculations were performed by firs t characterizing all the critical points of the system using ab initio comp lete active space-self consistent field (CAS-SCF) methods. These preliminar y calculations were followed by multireference configuration interaction (C I) calculations covering a grid of 1748 nuclear geometries. The surface has been fitted to a global analytic form, using the recently proposed Reprodu cing Kernel Hilbert Space Method of Ho , [J. Chem. Phys. 104, 2584 (1996)] and preliminary vibrationally-rotationally inelastic scattering cross secti ons have been computed using infinite order sudden quantum scattering metho ds. The motivation of the research is the evaluation of these inelastic cro ss sections which are needed in the modeling of circumstellar SiO maser rad iation. Significant structure has been observed in the computed inelastic c ross sections. This structure is of the form which might be expected to giv e rise to the vibrational-rotational quantum state population inversions wh ich in turn leads to the observed circumstellar maser action. An interestin g feature of the computed potential energy surface is a line of maxima in n onlinear geometries. These maxima appear to arise from a nonsymmetry relate d conical intersection. (C) 1999 American Institute of Physics. [S0021-9606 (99)00732-1].