Valence bond approach to the Pariser-Parr-Pople Hamiltonian and its application to simple π-electron systems

Abstract

The valence bond (VB) method is implemented for the Pariser-Parr-Pople (PPP) type hamiltonians, using the Clifford algebra unitary group approach (CAUGA) and various overlap-enhanced atomic orbital (AO) basis sets. It is shown that, as in the generalized and spin-coupled VB approaches, the choice of an appropriate AO-like basis is crucial to the success of the classical VB description. The method is first applied to π-electron systems of cyclobutadiene and benzene, for which different overlap-enhanced bases are investigated for coupling constants ranging from the fully correlated to the uncorrelated limit. Using a selection of 21 π-electron systems (from the 70 considered), for which exact solutions are readily available, the performance of the simplest version of this PPP-VB method, employing a single-parameter, nearest-neighbor overlap-enhanced AO basis, was investigated using a spectroscopic parameterization of the PPP hamiltonian. Already this simplest version of the PPP-V13 approach provides very good total π-electron energies, even when only covalent (or in most cases Kekulé) structures are employed. The transferability of the resulting optimal bases from one system to another is demonstrated and discussed.