Coupled ab initio potential energy surfaces for the reaction Cl(2P)+HCl→ClH+Cl(2P)

We have constructed the 1 (2)A', 2 (2)A' and 1 (2)A " potential energy surfaces for the Cl(P-2) + HCl --> ClH + Cl(P-2) reaction, together with the non-adiabatic coupling surface between the 1( 2)A' and 2 (2)A' states. All our calculations used the MOLPRO quantum chemistry package, with Dunning's correlation consistent augmented valence triple zeta 1-electron basis set. The 1 (2)A' and 1 (2)A" energies are calculated at the restricted open shell coupled cluster singles doubles with perturblative triples (RCCSD-T) level, whilst the 2 (2)A'-1 (2)A' energy difference and the non-adiabatic coupling are calculated via the multireference configuration interaction (MRCI) technique. The non-adiabatic coupling is evaluated from transition matrix elements of the angular momentum operator, namely [1 (2)A " \(L) over cap\ 1 (2)A'] and [1 (2)A " \(L) over cap(x)\ 2 (2)A']. The surfaces in a diabatic representation are fitted to rotated-Morse cubic-spline functions. The empirical long-range potentials of Dubernet and Hutson (J. Phys. Chem., 1994, 98, 5844), together with empirical short range potentials, are then combined with the fitted ab initio surfaces to produce a set of global potential energy surfaces. Convergence tests show that the height of the barrier at C-2v geometries is 0.4361 eV for the 1 B-2(1) state, and occurs at a ClHCl bond angle of 137 degrees. The collinear barrier heights are 0.4939 eV on the (2)Sigma(u)(+) surface and 0.9416 eV on the (2)Pi(g) surface.