Metal-insulator transition and charge ordering in the extended Hubbard   model at one-quarter filling
  We study with exact diagonalization the zero temperature properties of the quarter-filled extended Hubbard model on a square lattice. We find that increasing the ratio of the intersite Coulomb repulsion, $V$, to the band width drives the system from a metal to a charge ordered insulator. The evolution of the optical conductivity spectrum with increasing $V$ is compared to the observed optical conductivity of several layered molecular crystals with the theta and beta'' crystal structures. 
M. Calandra (1), J. Merino (1) and Ross H. McKenzie (2)   ((1)Max-Planck-Institut, (2)University of Queensland)