Correlation effects of IT electrons. II. Low-lying excitations of polycyclic hydrocarbons

We show how the Coulomb interaction between π electrons in a conjugated molecule is screened by the presence of the other π electrons. We describe how the resulting effective interaction can be calculated and used in a configuration-interaction calculation to determine the low-lying excitations of such molecules. The methods are applied to the study of a series of hydrocarbons from benzene to hexabenzocoronene and good agreement with the experimental spectra is obtained. The inclusion of these correlation effects predicts a characteristic nommiform distribution of charge in alternant hydrocarbons.