Numerical study of a Projected Hubbard Model

The extension of a generalized Hubbard model, recently introduced by A. Montorsi and M. Rasetti, is studied by means of numerical diagonalization, Quantum Monte Carlo and variational methods. Exact results for lattices up to 4 x 4 are presented. Quantum Monte Carlo simulations reproduce our exact numerical results. We use these data to assess the accuracy of variational (including generalized BCS like) methods.