Exact Ground State of Strongly Correlated Electron Systems from   Symmetry-Entangled Wave-Functions
  The four-site Hubbard model is considered from the exact diagonalisation and variational method points of view. It is shown that the exact ground-state can be recovered by a symmetry projected Slater determinant, irrespective of the interaction strength. This is in contrast to the Gutzwiller wave-function, which is calculated as well. 
A. Lepr\'evost, O. Juillet and R. Fr\'esard
cond-mat.str-el