Configurational interaction in molecular orbital theory. A higher approximation in the non-empirical method

Abstract

The disagreements between calculations of the benzene energy states by the method of antisymmetric molecular orbitals (A. S. M. O.) and by the augmented valence-bond theory given in previous papers (Craig 1950 a to c ) are due, at least in part, to the neglect in the former of interaction between different ‘configurations’. Every scheme of assigning electrons to molecular orbitals is a configuration, and where states with different configurations have the same symmetry they interact with one another under the influence of electron repulsion and suffer energy changes. In this paper a study of these energy changes is made, using a limited set of configurations. The A. S. M. O. states are affected by amounts up to 5 eV, and this is enough to show that the A. S. M. O. theory cannot be used for the detailed understanding of molecular spectral bands. The inclusion of configurational interaction gives the next higher approximation and makes the method, in principle, as general as is compatible with the initial use of 2 p atomic wave functions. The method ought to prove valuable for settling in a non-empirical way issues for which the correctness of the simple empirical methods is in doubt. Even in the improved theory, however, the numerical precision required to interpret spectral separations in the order of 1 eV will be difficult to attain without the use of three-centre integrals and some other refinements.