Potential energy surfaces for the 3A" and 3A′ electronic states of the O(3P) plus HCl system

We report ab initio calculations at the MRCI+Q/CBS level of theory for the (3)A" and (3)A' electronic states of the O(P-3)+HCl system, where the complete basis set energies are obtained by extrapolating MRCI+Q/aug-cc-pVnZ (n=2,3,4) energies. Potential energy surfaces for these electronic states are generated by interpolating these energies using the reproducing kernel Hilbert space method. The reaction barrier on the interpolated (3)A" surface at the MRCI+Q/CBS level of theory is 11.86 kcal/mol. The potential energy surface was then scaled to yield a barrier height close to that predicted by CCSD(T) and MRCI+Q benchmark calculations, namely, 10.60 kcal/mol. The fact that the (3)A" and (3)A' electronic states are degenerate at collinear and asymptotic regions of configuration space was used to scale the collinear reaction barrier on the (3)A' surface from the MRCI+Q/CBS value of 15.15 kcal/mol to match that of the scaled (3)A" surface, 13.77 kcal/mol. The potential energy surfaces thus obtained appear to be the most accurate to date for the reaction O(P-3)+HCl-->OH+Cl. The potential energy surface for the (3)A" state contains a fairly deep van der Waals well on the product side of the reaction barrier at a rather sharp O-H-Cl angle (67degrees) and a shallow well on the reactant side at collinear O-H-Cl geometry. Details of the ab initio calculations, the fitting procedure, and characterization of the saddle and stationary points are presented. (C) 2003 American Institute of Physics.