The potential energy surface for the F+H₂ reaction as a function of bond angle in the saddle point vicinity

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Møller-Plesset fourth order perturbation theory) calculations of the FH₂ potential energy surface for collinear and bent geometries in the vicinity of the F---H-H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict.