The development of a new hybrid (QM/MM) method, where the QM part is treated by ab initio valence bond (VB) theory is presented. This VB/MM method has the advantages of empirical VB (EVB) methodology but does not rely on empirical parameterization for the quantum part. The method implements embedding of the quantum region of each diabatic state separately, by treating the electrostatic interactions between QM and MM regions classically. Additionally, it assumes that changes of the off diagonal matrix element due to different environments are such that the overall resonance integral does not change. These assumptions are discussed in detail and the validity of the method is shown to be successful in three different bond dissociation processes, where bond dissociation as well as solvation energies compare very well with the experimental data.

The transition metal hydride cations, TMH+ (TM = first transition metal row, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn), have been studied using valence bond (VB) theory to elucidate the bonding in these systems through VB concepts. Although the bonds appear extremely covalent by virtue of charge distribution, this appearance conceals key contributions to bonding, such as covalent-ionic resonance energy (RECS) and relaxation energy of the inactive electrons (ΔE relax(inactive)). The RECS term is seen to increase from ScH+ toward ZnH+, becoming significant in the late TMH+ molecules. The ΔE relax(inactive) term, which accounts for the nonbonding 3d n electrons and the 3s23p6 core electrons, is always significant. Furthermore, for all of the bonds from CrH+ to CuH+, the relaxation term makes a major contribution to the bond energy. It appears therefore, that in these TM−H+ bonds, the spin pairing of the bonding electrons can act as a trigger for the nonbonding and adjacent core electrons to relax their Pauli repulsion and thereby strengthen the binding of TM+ and H. As a result of the general weakness of TM bonds, the relaxation is expected to frequently be an important bonding contribution. The major function of the inactive and core electrons shows that the traditional role of “covalency” must be reassessed in a systematic manner.