Ab initio calculation and quasi-classical dynamics study of the two lowest potential energy surfaces of the O(1D) plus HBr system

The lowest singlet 1(1)A ' and 1(1)A potential energy surfaces (PES) of the O(D-1) + HBr system have been ab initio computed. The complete active space self-consistent field (CASSCF) method was used in most of the calculations, considering all the valence orbitals as active. The calculations were complemented with both analytical gradient calculations to characterize the stationary points and multireference configuration interaction (MRCI) calculations at selected nuclear geometries to improve the determination of the barrier heights and of the energetics. Electronic energy values for both PESs were then independently fitted by polynomial expansions in bond order coordinates. On the fitted surfaces quasi-classical trajectories were separately run. Single-surface calculations behave qualitatively different for the ground and the excited PES at low collision energies. A satisfactory agreement with existing experimental data was obtained by using the ground PES while calculations performed on the excited 1(1)A PES worsened the agreement. However, when collision energy is increased, detailed experimental distributions are less well reproduced by calculations on the ground PES. This may imply the participation via nonadiabatic transitions of the 2(1)A ' PES at higher energies while the adiabatic ground singlet PES well describes reactive scattering at low collision energy. (C) 2002 John Wiley & Sons, Inc.