Generalized Hartree-Fock theory and the Hubbard model

@article{Bach1993GeneralizedHT,
  title={Generalized Hartree-Fock theory and the Hubbard model},
  author={Volker Bach and Elliott H. Lieb and Jan Philip Solovej},
  journal={Journal of Statistical Physics},
  year={1993},
  volume={76},
  pages={3-89}
}
The familiar unrestricted Hartree-Fock variational principles is generalized to include quasi-free states. As we show, these are in one-to-one correspondence with the one-particle density matrices and these, in turn, provide a convenient formulation of a generalized Hartree-Fock variational principle, which includes the BCS theory as a special case. While this generalization is not new, it is not well known and we begin by elucidating it. The Hubbard model, with its particle-hole symmetry, is… 

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