Comparison of mean-field theories of an extended Hubbard model

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Abstract
Models of strongly interacting Fermi systems are sufficiently intractable that simple mean-field approximations (typically Hartree-Fock) are often employed to obtain qualitative insight into their properties, and also to set parameter values by comparing with band-structure calculations. However, if the correlations are accounted for with use of more sophisticated (but still mean-field-like) methods, qualitative changes in charge densities and magnetic properties can result. We compare four ‘‘mean-field’’ approximations for an extended Hubbard model and show that they yield quite different results. Implications for the choice of model for high-temperature superconductors are discussed.