Ab initio valence bond study of the origin of barriers to hydrogen exchange reactions: Application of the valence bond self-consistent-field method to the F + HF → FH + F reaction

A combination of ab initio methods, in particular the valence bond self-consistent field (VB-SCF) and the valence bond configuration interaction (VB-CI) methods, are used to examine the origins of barriers to chemical reactions in a quantitative manner. Valence bond methods are well suited to this type of study as their underlying philosophy relies on constructing the molecular (or supermolecular) electronic wave functions from those of the constituent atoms or fragments. The valence bond self-consistent-field (VB-SCF) method permits the computation of optimized nonorthogonal orbitals for any desired combination of valence bond structures. While the general VB-SCF method permits a full optimization of the nonorthogonal orbitals, in the present case, the optimization is restricted so that the orbitals retain their "atomic" character. A full optimization results in the final optimized nonorthogonal orbitals possessing contributions from several atomic centers. This delocalization leads to lower energies but prevents the qualitative characterization of the valence bond structures into "reactant" and "product" structures as (if full optimization is used) the orbitals span the entire supermolecule. Comparisons are made between VB-SCF, VB-CI, and CAS-SCF computations using the same basis set. The model problem examined here is the reaction F^• + HF → FH + ^•F. The reaction is discussed in terms of noninteracting or "diabatic" reactant and product potential energy curves. The "curve crossing" picture of the origin of barriers to chemical reactions maintains that the "true" wave function of the system changes smoothly from a reactant to a product wave function as the reaction proceeds. This picture is examined through quantitative calculations in the present work, and its validity is critically tested.