The Valence Bond Orbital Model as an Interpretive Framework for Understanding Electronic Structure

For purposes of interpretation and understanding, the valence bond orbital model of electronic structure provides a number of advantages over the more common molecular orbital model. This derives primarily from the unique description of valence bond orbitals in terms of highly localized bond pairs, lone pairs, etc. Optimization of the orbitals by a self‐consistent‐field procedure brings out features which were not evident from the semi‐empirical treatments of earlier workers. Calculations on CH4, NH3, H2O and H2S show that optimized valence bond orbitals are often bent even at the equilibrium nuclear configuration and generally do not follow the nuclei when bond angles are varied. Analysis of a large basis set calculation on H2O shows that the valence bond wavefunction can be fitted well with a minimal basis, making it possible to interpret more clearly the connection between hybridization and interorbital angles. It is shown how the valence bond method provides a rigorous basis for factorization even of difficult second order physical properties, such as polarizability, into separated bond and lone pair contributions. These contributions can be further decomposed into sums of atomic valence state values plus corrections due to bond polarization effects. Finally, the advantages of using the valence bond model as a starting point for the perturbation treatment of electron correlation are pointed out. Calculations on H2and LiH indicate that valence bond theory need only be carried through second order to obtain results comparable in accuracy to a third order treatment based on molecular orbitals.