THE CORRECTION VECTOR APPROACH TO LINEAR AND NONLINEAR-OPTICAL PROPERTIES OF CONJUGATED SYSTEMS

The sum-over-states (SOS), time dependent perturbation theory expansio n over molecular eigenstates method for the computation of molecular h yperpolarizabilities is briefly reviewed. A correction vector method f or the computation of linear and nonlinear optical properties of pi-co njugated systems has been devised for use with a singly and doubly exc ited configuration interaction model (SDCI). The SOS approach and the correction vector method are equivalent when the same basis set of man y-electron configurations is used in both. It has been verified that t he values obtained from the correction vector method and by direct sum mation over eigenstates are identical. The correction vector method ha s been found to be very efficient for larger systems, both in terms of central processing unit (CPU) time and storage requirements. The dire ct SOS summation has been used to study the approach to the final SDCI values as excited states are added in order of increasing energy. Usi ng the methods described in the previous paragraph, a study of the pol arizabilities and hyperpolarizabilities for frequency doubling and tri pling in unsubstituted polyenes and in amino-, nitro-, donor/acceptor substituted polyenes, and 4-nitroaniline (pNA) has been made. The basi s set has been restricted to singly and doubly excited configurations between pi orbitals (piSDCI) which have been extracted from the ground state obtained from a complete neglect of differential overlap (CNDO) calculation. The results are found to be in good agreement with other values reported in literature. They also show that the full piSDCI hy perpolarizabilities of the polyenes are largely accounted for by the c ontributions of a small number of low-lying excited states, tending to justify the use of simplified models such as the two and three state models in these systems.