Abstract
The thermodynamical potential of Hubbard models extended by either nearest-neighbour Coulomb correlation and/or nearest-neighbour Heisenberg exchange is calculated analytically for a triangle and tetrahedron. It is shown that the various degeneracies of the grand-canonical energy levels are partially lifted by the exchange interaction and completely by the Falicov term. The influence of the additional interactions on the specific heat has been calculated for the tetrahedron.
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