Refined N.P.S.O. semi-empirical treatment of benzene

Abstract

A treatment of the lower excited levels of benzene has been carried out within the framework of the method of non-paired spatial orbitals (N.P.S.O.), using the semi-empirical parameters characteristic of the Parser-Parr approach. Parameters have been introduced which allow the physical form of the N.P.S.O. function to be preserved, while using a basis set of orbitals for which the central approximation of zero differential overlap is reasonably valid. Although “ionic terms” are considerably enhanced in the Pariser-Parr treatment the N.P.S.O. function accommodates the change and takes account of a considerable part of the correlation energy.