Ground state energy of the Hubbard model: cluster-perturbative results
  When electron correlations are important it is often necessary to use numerical methods to solve the Hamiltonian for a finite system (cluster) "exactly". Unfortunately, such methods are restricted to small systems. We propose to combine the "exact" numerical diagonalization for small clusters with the perturbative calculations to take into account the intracluster as well as intercluster interactions. 
Maciej M. Maska