Electron correlations in narrow energy bands - IV. The atomic representation

Abstract A reformulation and extension of the previous theory is described. The new formulation has the advantage of being conceptually simpler than the earlier theory; it can, in fact, be developed in close analogy with the usual theory of spin waves. It is shown that the simplest approximations in the new formulation lead to theories of the crystal field, the Weiss molecular field, spin waves, polarization waves and the quasi-electron theory of the earlier papers of this series. The new formulation permits one to take into account the interactions of electrons on different atoms and to consider solutions of the anti-ferromagnetic and more complicated kinds. The theory is developed by the ‘equation of motion’ method.