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

The sum-over-states (SOS), time dependent perturbation theory expansion over molecular eigenstates method for the computation of molecular hyperpolarizabilities is briefly reviewed. A correction vector method for the computation of linear and nonlinear optical properties of pi-conjugated systems has been devised for use with a singly and doubly excited configuration interaction model (SDCI). The SOS approach and the correction vector method are equivalent when the same basis set of many-electron configurations is used in both. It has been verified that the values obtained from the correction vector method and by direct summation over eigenstates are identical. The correction vector method has been found to be very efficient for larger systems, both in terms of central processing unit (CPU) time and storage requirements. The direct SOS summation has been used to study the approach to the final SDCI values as excited states are added in order of increasing energy. Using the methods described in the previous paragraph, a study of the polarizabilities and hyperpolarizabilities for frequency doubling and tripling in unsubstituted polyenes and in amino-, nitro-, donor/acceptor substituted polyenes, and 4-nitroaniline (pNA) has been made. The basis 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 hyperpolarizabilities of the polyenes are largely accounted for by the contributions 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.