Correlation effects and charge fluctuations in .pi. systems: a combined PPP and localized many-particle analysis

A local correlation approach is employed to elucidate the many-particle character of the π electronic system in hydrocarbons, heterocompounds, and a collection of other π networks. The self-consistent-field (SCF) wave function |ψ SCF >, which is the prerequisite to derive the correlated ground state |ψ 0 >, is calculated in the Pariser-Parr-Pople (PPP) scheme. The matrix elements occurring in the SCF precursor as well as the many-particle part are calculated ab initio in a minimal basis of Slater-type atomic orbitals (AOs). In the determination of |ψ 0 >, singly and doubly excited configurations are considered. The former ones optimize the one-particle density beyond the results from the SCF calculation