The Quantum-Mechanical Treatment of Molecules by the Method of Spin Valence

Abstract

The method of spin valence, developed by Born, Heitler, Rumer and Weyl for the treatment of molecular systems, becomes completely equivalent to the Slater method if the bonding forces are considered to be exerted between individual orbitals, and not between entire atoms in definite spectroscopic states. This change in the point of view eliminates several difficulties which were formerly present in the method (e.g., the obscuring of the directional properties of the valence bonds). The secular equation, for which a simplified derivation is given, is of quite a different form from the Slater equation, and the problem of its solution is considered in some detail. In addition to the rigorous methods, several approximate methods are given, of which two (applicable to unsaturated and aromatic hydrocarbons) do not require that the secular equation be set up at all. The method of spin valence is found to be more convenient than the Slater method for the treatment of systems of high multiplicity. Finally, the extension to cases of orbital, as well as spin degeneracy is briefly outlined.