Electronic levels in simple conjugated systems III. The significance of configuration interaction

Abstract
The features of configuration interaction calculations in π-electron systems so far reported are analyzed and compared with some atomic, and diatomic molecular, systems. The interactions between configuration wave functions are not negligible in atoms and are important in molecules bound either by σ- or π-electrons. In the one case where comparison is possible, viz. in H2 (Coulson 1938), the magnitude of the effect is reduced to one-half when the configurations refer to self-consistent field orbitals rather than to linear combinations of atomic orbitals. The situation in which a consideration of configuration interaction is inescapable is for the configurations of a shell of molecular electrons where, typically, the spread of the energies is small compared with the total electronic energy. In cyclobutadiene, butadiene, benzene and naphthalene, i. e. in all cases so far reported, even the order of some of the states is changed. The wave functions derived in these calculations are markedly better than single-configuration ones, as judged by the much better agreement between calculated and experimental spectral transition probabilities.

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