An accurate multireference configuration interaction calculation of the potential energy surface for the F+H-2->HF+H reaction

A three dimensional potential energy surface for the F+H-2-->HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self-consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple-zeta plus polarization basis set employed in previous nine-electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H-2 fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h functions on fluorine, a linear barrier height of 1.84 kcal/mol and an exothermicity of 31.77 kcal/mol (exp. 31.73 kcal/mol) was obtained. The true saddle point has a bent structure and the barrier height is predicted to be (1.45+/-0.25) kcal/mol. About 700 points on the three-dimensional potential energy surface have been computed using a slightly smaller basis set, which yield F-HH barrier heights of 1.92 kcal/mol (linear), 1.54 kcal/mol (bent), and an exothermicity of 31.3 kcal/mol. The barrier height for the H+FH-->HF+H exchange reaction is predicted to be 41.2 kcal/mol. In the entrance channel cuts through the three potentials correlating with F(P-2(3/2,1/2)) + H-2((1) Sigma(g)(+)) have been computed, and the effect of spin-orbit coupling is investigated. It is found that the spin-orbit coupling increases the barrier height relative to the asymptotic F(P-2(3/2)) + H-2((1) Sigma(g)(+)) ground state by about 0.35 kcal/mol, leading to final estimates for the effective collinear and bent barriers of (2.18+/-0.25) kcal/mol and (1.80+/-0.25) kcal/mol, respectively. An accurate global analytical fit of the potential (without the effect of spin-orbit coupling) has been obtained using the method of Aguado and Paniagua. Our new ab initio potential is compared to various potentials used so far in dynamics calculations for the F+H-2 reaction. (C) 1996 American Institute of Physics.