Two-photon spectroscopy of dipole-forbidden transitions. I. Dipole-forbidden transitions and double excited configurations in the CNDO-CI methods

Abstract

The applicability was investigated of CNDO/S-type methods for the calcn. of optical spectra of mols. with the special implication that the calcns. should not only describe the intense, dipole-allowed transitions which dominate the 1-photon absorption spectrum but also those transitions which are 1-photon forbidden in 1st order approxn. Such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if doubled excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still 2 types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approxn. used to describe the R-dependence of the Coulomb repulsion gains much more importance than in cases where mainly dipole allowed transitions are of interest. These findings confirm results obtained earlier from theories for which the pairing theorem is valid. The calcd. data show an excellent stability with respect to further increase of the no. of configurations if at least ~200 energy selected configurations are taken into account.