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 pi electronic system in hydrocarbons, heterocompounds, and a collection of other pi networks. The self-consistent-field (SCF) wave function \psi(SCF)], which is the prerequisite to derive the correlated ground state \psi0), 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 \psi0], singly and doubly excited configurations are considered. The former ones optimize the one-particle density beyond the results from the SCF calculation. The atomic pi-electron density localization is measured by the mean-square deviations of the electronic charge at center i [(DELTAn(i)2)]corr around the corresponding mean-value [n(i)]. The many-particle character of pi electron bonding is quantified by two correlation-strength parameters DELTA(i) and SIGMA(i), mapping the reduction in the charge fluctuations [(DELTAn(i)2]corr, due to electronic correlations. It is shown that the pi electronic localization properties sensitively depend on the topology of the considered pi center and the type of the atom. Pi systems are generally far from the independent-particle limit. Stronger electronic correlations lead to sizeable atomic localization of the pi electrons. Heteroatoms on the right-hand side of C within the periodic table cause enhanced pi electron correlations. Bond-length alternation is accompanied by an enhancement of the electronic charge fluctuations [(DELTAn(i)2)]corr; the delocalization of the pi electrons is thereby strengthened. This [(DELTAn(i)2)]corr enhancement as a function of the bond-length alternation is an increasing function of the correlation strength operative in the pi network.