Limiting transition from the model of collective motion of π-electrons in molecules to the semiempirical Pariser-Parr-Pople theory
In the context of a two-parameter scheme (correlation and localization) we examine the region of applicability of various π-electron theories. The limiting transition from the model of collective motion (MCM) to Pariser-Parr-Pople (PPP) theory shows that PPP theory is inconsistent. The PPP model Hamiltonian derived in the approximation of strong localization of π-electrons has been invalidly extended to the region of weak localization, thus losing an important branch of low-lying collective π-electron excitations. Inclusion of this branch qualitatively alters the results of PPP theory. Furthermore, the conclusion of PPP theory that there are no singlet excitations of the benzene molecule below 4.7 eV conflicts with several experiments.