Study of the electronic states of the benzene molecule using spin-coupled valence bond theory

Abstract

The spin-coupled VB method is used to study all the singlet and triplet valence excited states, as well as the n=3,4 singlet and triplet Rydberg states of benzene below the first ionization potential at 9.25 eV. The valence excited states are classified in an obvious physical way into covalent or ionic states, from which it follows at once that covalent states are well described using the approximation of σ/π separation and a frozen σ core, whereas the error in the computed transition energies to the ionic states is much larger and these states require additional σ/π correlation for their proper description. The Rydberg states are very well-described, provided that a suitable σ core, derived from a calculation on the C6H+6 ion, is used. The numerical accuracy of the final results for the transition energies is at least the same as that given by the largest MO-CI- or CASSCF-CI-based methods reported to date. The spin-coupled VB approach has the obvious advantage in providing a compact and clear picture of the various states.