Convergence of valence bond and molecular orbital theories

Abstract

A comparison of the rate of convergence of valence bond (VB), symmetrically orthogonalized-orbital valence bond (), and configuration-interaction molecular orbital (SCF) calculations has been made for model π-electron systems in the minimum basis set approximation; the allyl radical, trans-butadiene, and the methylene cyclobutadienyl radical were studied. For excited states, as well as for the ground state, it is found that VB provides the best convergence in most cases and yields the worst results, with SCF typically falling in between. Properties examined include energy eigenvalues, charge and spin densities, transition energies, and transition dipole moments. Charge-density convergence is in opposite directions for the VB and SCF methods, with VB (and ) overemphasizing the charge on the atoms and SCF overemphasizing the charge in the bonding regions.