LCAO MO Procedure for Determining π-Electron Screening and the Low-Lying Excitations of Conjugated Hydrocarbons

A procedure is described whereby the effective electron–electron interaction in a large conjugated molecule is determined by considering the screening of the field of an artificial test charge imbedded in the molecule due to the redistribution of the $\pi$ electrons. It is argued that the form of this static screened Coulomb interaction should represent well the effective interaction between electrons near the Fermi energy. This is tested by determining this screened Coulomb interaction in hexabenzocoronene containing an artificial test charge using the standard SCF MO method of calculation, and then using this screened interaction as the effective interaction between the $\pi$ electrons to determine the low‐lying excitation of the molecule with the test charge removed. Agreement between the predicted spectrum and the experimental spectrum is excellent and significantly better than that obtained with either the Pariser–Parr semiempirical expression or the use of Nishimoto and Mataga's expression for the effective interaction. A calculation of the same type of coronene gives similar excellent agreement with experiment.