Cellular Dynamical Mean Field Approach to Strongly Correlated Systems
  We propose a cellular version of dynamical-mean field theory which gives a natural generalization of its original single-site construction and is formulated in different sets of variables. We show how non-orthogonality of the tight-binding basis sets enters the problem and prove that the resulting equations lead to manifestly causal self energies. 
Gabriel Kotliar, Sergej Y. Savrasov, Gunnar Palsson