Pariser—Parr Theory: Quantum Mechanical Integrals from the Benzene Spectrum

Abstract

The integrals which occur in Pariser—Parr ASMO—CI theory for carbon atoms at short internuclear distances (≤2.40 Å) can be derived from the spectrum of benzene [R. Pariser, J. Chem. Phys. 24, 250 (1956)]. Previously, various workers have uniformly neglected the highest singly excited configuration in the configuration interaction on the grounds of its high energy and because its inclusion destroys the simple linear formulas for the state energies of benzene. The effect of inclusion of this configuration on the spectral matching method of determining nearest‐neighbor β_pq, γ_pq, and γ_pp, employing the ¹B_2u, ¹B_1u, ¹E_1u, and ³B_1u states of benzene has been investigated. The major effect was lowering of [11 | 11] to a value near that expected theoretically when correlation effects are considered. The influence of the new parameters on spectral calculations was explored for benzene, cis‐ and trans‐butadiene, naphthalene, and azulene, and the major effect observed was in the energies of excited triplets. It is suggested that this set of parameters, derived consistently and solely from electronic spectra, will be useful for spectral calculations.