Potential energy surfaces for the 3 A ″ and 3 A ′ electronic states of the

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 ( 3 P) + HCl system, where the complete basis set (CBS) 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 (RKHS) 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 ( 3 P) + 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 (67°) 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