A πCI approach to the study of correlation effects on the nonlinear-optical properties in organic π-conjugated systems

Abstract

A π-electron method which allows for the systematic inclusion of configuration interaction of any order has been developed for the computation of electronic and optical properties of conjugated molecules. It has been used to study the effect of electron correlation on these properties in all trans finite polyenes of up to 16 carbon atoms. For smaller molecules it has been possible to carry out a complete set of CI calculations, from singly excited (SCI) to full configuration interaction (FCI). For the larger molecules the SCI and doubly excited CI (SDCI) calculations have been performed. The program permits the execution of a configuration interaction calculation of any order, n, in which all configurations involving the excitation of 1,2,...,n electrons from the occupied π-orbitals of the Hartree–Fock ground-state to the virtual π-orbitals are included. The set of π-orbitals is extracted from the ground state obtained from an all valence-electron, complete neglect of differential overlap (CNDO) calculation. The configurations are represented by binary integers so that their generation and storage is very rapid and efficient. The nonlinear optical properties have been computed mainly by the correction vector method but in some cases the sum-over-states (SOS) method has also been used to study the evolution of the THG coefficient as virtual states of increasing energy are added. The results obtained for the finite polyenes are found to be in very good agreement with both experimental and other theoretical values in literature. The results clearly show the effect of electron correlation, which is found to affect the electronic and optical properties of these systems both qualitatively and quantitatively.