Some Properties of Semiempirical Hamiltonians

Abstract

The Hamiltonian used in semiempirical Pariser—Parr—Pople (PPP)-type treatments is put in a form such that (a) the parameters on which correlation effects depend become obvious, and (b) the part of the Hamiltonian responsible for deviations from pairing properties of conjugated heterocycles with alternant topology is isolated. A set of approximate rules based on this analysis is derived. The rules concerning the extent of correlation effects are compared for some conjugated hydrocarbons with computations by full configuration interaction in the π-electron approximation. Generalizations are derived of known conditions under which heterocyclic systems are paired. The character of the two first excited states of heteroanalogs and heteroderivatives of alternant hydrocarbons is discussed, and rules determining deviations from the behavior of parent alternant hydrocarbons are formulated. This makes it possible to predict the polarizations of the first two transitions in these compounds in the PPP approximation with limited configuration interaction. The predictions are compared with some results of calculations on azines and amines.