The effect of the choice of parameters on the order of energy levels of benzene calculated in the π-electron approximation by the configuration interaction method including double- and triple-excited configurations

Abstract

A study is reported of the effect of the choice of parameters on the character of the spectrum of benzene calculated in the π-electron approximation by the configuration interaction method including double- and triple-excited configurations. Only the integrals which are not neglected in the usual versions of the Pariser, Parr, and Pople method have been taken into account. It is confirmed that the order of levels in the spectrum, in particular for the lowest E2g and B1u levels, depends markedly on the choice of parameters. We find that a simple expression permits an a priori prediction, from the parameter set used, of the order of the lowest E2g and B1u energy levels resulting from configuration interaction including higher excited configurations. The effect of the inclusion of triple-excited configurations can be neglected only in the case of the ground state (cf. [16]). However, it does not change the over-all character of the spectrum.