Ab Initio SCF MO and CI Calculations on the Electronic Spectrum of Benzene

An ab initio SCF MO calculation for the ground state of benzene is reported (total energy equal to −230.3745 hartrees). The set of SCF MO's resulting from this calculation is subsequently employed as the basis for a series of CI treatments investigating the theoretical description of the benzene electronic spectrum. It is found that the inclusion of double and higher excitation configurations in the CI treatment produces significant changes in the calculated results relative to those obtained in the more conventional single excitation CI. Perhaps the most important effects of the higher excitation configurations are observed in the class of excited singlet states; the addition of these configurations causes the B2u1 state to be lower than at least one of the three lowest benzene triplets, in agreement with experimental findings, and also leads to a considerable stabilization of the E2g1 species, making it the second most stable excited singlet in definite contrast with single excitation CI results.