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 excited 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 is presented in this paper in the frame of the semi-empirical CS-INDO method. With localized sigma MO's, only a few excitations are strongly coupled to the reference configurations. We show that they may be selected a priori on the basis of a topological criterion concerning the pairs of localized sigma MO's. These topological approximations induce a very small error on the low-lying singlet excited states of naphthalene and DMABN. Our results for these two molecules are in good agreement with experimental data and with the results of ab initio calculations. (C) 1997 Elsevier Science B.V.