Configuration-Interaction Wavefunctions for Small Pi Systems

Abstract

Complete CI wavefunctions for the pi electrons in allyl, pentadienyl, butadiene⁺, and butadiene⁻ are obtained in the context of the Pariser–Parr–Pople approximation. Because the basic set of Slater determinants is built up from atomic orbitals rather than molecular orbitals, it becomes trivially easy to set up the secular equation, and indeed many of the matrix elements required are zero. A comparison of the results obtained here with those found using more approximate methods indicates that the most serious discrepancies arise in the calculation of pi spin densities. The best of the approximate methods for calculating spin densities turns out to be a variant of the unrestricted Hartree–Fock (UHF) method.