Spin-coupled valence bond theory

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Abstract
In the spin-coupled description of molecular electronic structure, an N-electron system is described by N distinct—but non-orthogonal—orbitals, whose spins are coupled to the required resultant S in all possible ways. The coefficients of the basis functions comprising the orbitals and the coefficients of the different spin functions are fully optimized. The orbitals are frequently highly localized, and hence the model incorporates considerable electron correlation while retaining a high degree of visuality. The spin-coupled wave function is refined by non-orthogonal configuration interaction, and the final wave functions are of high quality but very compact. The various aspects of this theory are illustrated by a series of examples of increasing complexity: the H2 molecule, the BeH molecule, the 3B1 and lA1 states of CH2 and the cycloaddition of CH2 to ethenes, the 7t-electron system of benzene, and diazomethane (CH2N2). The results provide clear descriptions of the electronic structure and the associated processes. In benzene, the six π orbitals are highly localized, with far-reaching implications for the description of aromatic systems. The case of diazomethane shows that the central N atom takes part in fiv. electron-pair bonds, and the same is true for a series of molecules such as N2O, Hcno, NO2, and CH2NHO (nitrone), whose structures have long caused problems in valency theory.

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