Cluster expansion analysis for delocalized systems

Abstract The general formulas which are convenient for cluster analysis of a configuration interaction wave function are presented. These formulas are then used for cluster analysis of the “complete” configuration interaction wave functions of the π‐electronic models of benzene and butadiene obtained with a semiempirical method of the Pariser–Parr–Pople type using three different parameterizations. For butadiene the calculations are carried out with Hückel, Hartree–Fock, and Brueckner molecular orbitals. The results clearly indicate that Sinanoǧlu's statement 1, 2, concerning the relative unimportance of the linked parts of the tetraexcited state coefficients in the ci expansion, is justified for these delocalized systems.