Multireference configuration-interaction potential surfaces for the collinear F+H2 reaction

Abstract

Potential‐energy surfaces are reported for the collinear FHH system, using a multireference configuration interaction calculation combined with the rotated Morse curve‐cubic spline technique to fit the ab initio points. Basis A contains only nuclear‐centered atomic orbitals, while basis B adds bond functions at the midpoints of the FH and HH bonds. Results using basis B show significant improvement in the diatomic asymptotes, including the spectroscopic constants for H₂ and HF, and the reaction exoergicity. The FHH surface data are fitted along each ray in the (θ,l) coordinate system using either three‐parameter (GMF3) or five‐parameter (GMF5) generalized Morse functions. The five‐parameter fits are more accurate over a wider range, but the three‐parameter fits are good enough for most purposes. Based on the five‐parameter fits, surfaces A5 and B5 show the following properties (experimental values in parentheses): exoergicity 28.9 and 31.6 kcal/mol (31.7), barrier height 4.0 and 1.8 kcal/mol (1.5–2.3 kcal/mol), barrier location R_FH, R_HH=(1.47 and 0.77 Å) and (1.48 and 0.77 Å), and vibration frequencies (3550 and 720i cm⁻¹) and (3500 and 830i cm⁻¹). In addition, surface B5 contains reactant and product wells of depth 0.24 and 0.16 kcal/mol, respectively. The surface features obtained using basis B are consistent with the best current estimates obtained from either experimental or theoretical values, except for the location of the barrier which occurs somewhat later along the reaction coordinate.