Ab initio configuration interaction studies of the π-electron states of benzene

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Abstract
Ab initio configuration interaction studies of the π-electron states of benzene were carried out with a double-ζ basis set of contracted Gaussian functions augmented by two diffuse π functions on each carbon atom. The core potential of the σ electrons was obtained from an all-electron SCF calculation on the ground state. Vertical excitation energies of 5.00, 7.64, and 8.34 eV were obtained for the 1B2u, 1B1u, and 1E1u states, respectively, corresponding to the e1g→e2u excitation; the first two of these levels have a valencelike electron distribution, but the 1E1u state was found to have a diffuse 1e2u natural orbital, with = 38 bohr2. The analogous set of triplet states, all of which are valencelike, have calculated vertical excitation energies of 3.83 (3B1u), 4.98 (3E1u), and 7.00 eV (3B2u). The low-lying valence states 1E2g (8.33 eV) and 3E2g (7.28 eV) were found to have substantial double-excitation character, but no additional valence E2g states were obtained. Many other states, including quintets, Rydberg levels, and positive ion states, were computed and correlated with experimental data, where available. Discrepancies between computed and experimental energy levels are probably due in large measure to the frozen σ core approximation employed in this work, though basis set limitation may also play a part in some cases. An examination of the characteristics of the computed wavefunctions does not support some of the concepts used as a basis for semiempirical π-electron theories.