LOCALIZATION OF SIGMA-MOLECULAR-ORBITALS - TOWARDS A BETTER DESCRIPTION OF THE ELECTRONIC EXCITED-STATES OF LARGE CONJUGATED MOLECULES

The high number of two-electron integrals and the high number of excit ed configurations set severe limitations to configuration interaction (CI) calculations in large conjugated molecules. In order to get past such limitations, we perform the CI with delocalized pi and localized sigma molecular orbitals (MO's). This strategy is quite general, but i s presented in this paper in the frame of the semi-empirical CS-INDO m ethod. With localized sigma MO's, only a few excitations are strongly coupled to the reference configurations. We show that they may be sele cted a priori on the basis of a topological criterion concerning the p airs of localized sigma MO's. These topological approximations induce a very small error on the low-lying singlet excited states of naphthal ene and DMABN. Our results for these two molecules are in good agreeme nt with experimental data and with the results of ab initio calculatio ns. (C) 1997 Elsevier Science B.V.