Quantification of correlations in quantum many-particle systems
  We introduce a well-defined and unbiased measure of the strength of correlations in quantum many-particle systems which is based on the relative von Neumann entropy computed from the density operator of correlated and uncorrelated states. The usefulness of this general concept is demonstrated by quantifying correlations of interacting electrons in the Hubbard model and in a series of transition-metal oxides using dynamical mean-field theory. 
K. Byczuk, J. Kunes, W. Hofstetter, and D. Vollhardt
cond-mat.str-el quant-ph