Multireference configuration interaction calculations for the F(2P) plus HCl→HF+Cl(2P) reaction:: A correlation scaled ground state (1 2A′) potential energy surface

This paper presents a new ground state (1 (2)A(')) electronic potential energy surface for the F(P-2)+HCl -> HF+Cl(P-2) reaction. The ab initio calculations are done at the multireference configuration interaction+Davidson correction (MRCI+Q) level of theory by complete basis set extrapolation of the aug-cc-pVnZ (n=2,3,4) energies. Due to low-lying charge transfer states in the transition state region, the molecular orbitals are obtained by six-state dynamically weighted multichannel self-consistent field methods. Additional perturbative refinement of the energies is achieved by implementing simple one-parameter correlation energy scaling to reproduce the experimental exothermicity (Delta E=-33.06 kcal/mol) for the reaction. Ab initio points are fitted to an analytical function based on sum of two- and three-body contributions, yielding a rms deviation of < 0.3 kcal/mol for all geometries below 10 kcal/mol above the barrier. Of particular relevance to nonadiabatic dynamics, the calculations show significant multireference character in the transition state region, which is located 3.8 kcal/mol with respect to F+HCl reactants and features a strongly bent F-H-Cl transition state geometry (theta approximate to 123.5 degrees). Finally, the surface also exhibits two conical intersection seams that are energetically accessible at low collision energies. These seams arise naturally from allowed crossings in the C-infinity v linear configuration that become avoided in C-s bent configurations of both the reactant and product, and should be a hallmark of all X-H-Y atom transfer reaction dynamics between (P-2) halogen atoms. (c) 2006 American Institute of Physics.