COMPACT VALENCE-BOND FUNCTIONS WITH BREATHING ORBITALS - APPLICATION TO THE BOND-DISSOCIATION ENERGIES OF F2 AND FH

An original computational method of ab initio valence bond type is proposed, aiming at yielding accurate dissociation energy curves, while dealing with wave functions being very compact and clearly interpretable in terms of Lewis structures. The basic principle is that the wave function is allowed to have different orbitals for different valence bond structures. Thus, throughout the dissociation process, the so-called ''breathing orbitals'' follow the instantaneous charge fluctuations of the bond being broken by undergoing changes in size, hybridization, and polarization. The method is applied to the dissociation of F-2 and FH. For each molecule, a wave function involving only three valence bond configurations yields equilibrium bond lengths within 0.01 Angstrom, and dissociation energies within about 2 kcal/mol of the results of estimated or true full configuration interaction in the same basis sets. The effect of dynamical electron correlation on calculated dissociation energies is analyzed. It is shown that restricting the correlation to its nondynamical part results in an improper treatment of ionic terms due to a mean-field compromise in the optimization of the orbitals.