Inclusion of intersite spatial correlations in the alloy analogy   approach to the half-filled ionic Hubbard model
  Using the nonlocal coherent-potential approximation we study the effect of intersite spatial correlations on the transition from band insulator to metal as well as from metal to Mott insulator in the "alloy analogy" approach to the paramagnetic solution of the half-filled ionic Hubbard model. We find that intersite spatial correlations enhance the metallic phase. 
D. A. Rowlands and Yu-Zhong Zhang
cond-mat.str-el