Non-Orthogonal Wave Functions and Ferromagnetism

Close consideration of the influence of non-orthogonality of the electronic wave functions in a crystal shows that because of the large number of terms arising from the possible permutations of electrons, the usual procedure involving the neglect of most of the terms arising from this lack of orthogonality may be seriously in error. The dependence of the relative energies of the states of low and high multiplicity on this factor is discussed and exemplified in the cases of some simple molecular models.