MULTIREFERENCE CONFIGURATION-INTERACTION POTENTIAL SURFACES FOR THE COLLINEAR F+H-2 REACTION

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 H-2 and HF, and the reaction exoergicity. The FHH surface data are fitted along each ray in the (theta,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 angstrom) and (1.48 and 0.77 angstrom), 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.