Localized excitations in pyrazine and p-benzoquinone. A valence bond model

A valence bond (exciton) model involving the broken symmetry solutions to the Hartree–Fock equations provides reasonably accurate excitation energies for the 1,3(n→π*) excited states and the 2n ionic states of pyrazine and para-benzoquinone. An approach to the nonorthogonal CI is outlined which dramatically simplifies the calculation of the matrix elements.