Global ab initio potential energy surfaces for the ClH2 reactive system

Two new global ab initio potential energy surfaces (called BW1 and BW2) for the ClH2 reactive system are presented. These are based on internally contracted multireference configuration interaction calculations using a very large basis set, performed at 1200 geometries. Accurate analytical fits have been generated using the functional form proposed by Aguado and Paniagua. The BW1 surface is based on the original ab initio points. This surface slightly underestimates the dissociation energies of the diatomic fragments and overestimates the barrier height. Therefore, a second surface (BW2) has been computed by scaling the correlation energies at all geometries with a constant factor, which was chosen such that the dissociation energies of HCl and H-2 are reproduced more accurately. The barrier heights for the collinear transition state of the Cl+H-2--> HCl+H reaction are computed to be 8.14 kcal/mol and 7.61 kcal/mol for the BW1 and BW2 surfaces, respectively. To these values the spin-orbit correction of 0.84 kcal/mol has to be added, yielding a best estimate for the true barrier height of 8.45 kcal/mol. In the entrance channel of the Cl+H-2--> HCl+H reaction a T-shaped van der Waals well with a depth of 0.51 kcal/mol is found, while in the exit channel a van der Waals well with a collinear geometry and a depth of 0.45 kcal/mol is predicted. For the H+ClH exchange reaction, which also has a collinear transition state, the barrier heights are computed to be 18.5 kcal/mol and 17.9 kcal/mol for BW1 and BW2, respectively. It is shown that the topology of the new surfaces differs qualitatively from previous semiempirical surfaces, and the implications on the dynamics of the H-2+Cl reaction are discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)30201-X].