CORRELATION PROBLEMS IN ATOMIC AND MOLECULAR-SYSTEMS .7. APPLICATION OF THE OPEN-SHELL COUPLED-CLUSTER APPROACH TO SIMPLE PI-ELECTRON MODEL SYSTEMS

The coupled‐cluster variational‐like direct approach to the calculation of ionization and electron attachment energies and of excitation energies is applied to several π‐electron model systems, using the PPP Hamiltonian with various parametrizations. A simple approximation, which represents the triexcited clusters in terms of disconnected W1T2 terms, is employed. All the necessary diagrams for both excitation energy and ionization potential (electron affinity) calculations are given in the compact Hugenholtz nonoriented form. The results of the calculations for benzene, trans‐butadiene, all‐trans‐hexatriene, and fulvene are compared with the corresponding full CI results, and the conclusions about the validity and efficiency of this approach are drawn. Copyright © 1979 John Wiley & Sons, Inc.