MULTICONFIGURATIONAL 2ND-ORDER PERTURBATION-THEORY APPLIED TO THE CALCULATION OF ELECTRONIC-SPECTRA OF CONJUGATED SYSTEMS

A newly proposed method for the calculation of complex electronic structures in molecules is applied to the calculation of excited states. The method is a two step procedure: The Complete Active Space (CAS) SCF method is used to calculate the molecular orbitals and a reference function. This step thus takes care of the basic interactions including the coupling of nearly degenerate configurations, which is very common for excited states. Dynamic electron correlation effects are added in the second step, where second order perturbation theory is used with the CASSCF wave function constituting the reference function. The procedure has been successfully applied to a number of electronic spectra, yielding results of much higher accuracy than has been possible to obtain with traditional CI based methods. The lectures will give examples from studies of benzene, the azabenzenes, short polyenes, and three hetero-cyclic pentadienes. In several of these studies have the theoretical results provided a new understanding of the spectra and suggested a number of new assignments.