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

We have constructed the 1 ²A', 2 ²A' and 1 ²A'' potential energy surfaces for the Cl(²p) + HCl → ClH + Cl(²P) reaction, together with the non-adiabatic coupling surface between the 1 ²A' and 2 ²A' 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 ²A' and 1 ²A'' energies are calculated at the restricted open shell coupled cluster singles doubles with perturbative triples (RCCSD-T) level, whilst the 2 ²A'-1 ²A' 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 ²A'' |L̄(x)| 1 ²A') and (1 ²A'' |L̄(x)| 2 ²A'>. The surfaces in adiabatic 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 C₂(v) geometries is 0.4361 eV for the 1 ²B₁ state, and occurs at a ClHCl bond angle of 137°. The collinear barrier heights are 0.4939 eV on the ²Σ(u)⁺ surface and 0.9416 eV on the ²Π(g) surface.