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In the context of many-fermion systems, "correlation" refers to theinadequacy of an independent-particle model. Using "free" states as archetypesof our independent-particle model, we have proposed a measure of correlationthat we called "nonfreeness" [Int. J. Quant. Inf. 5, 815 (2007)]. Thenonfreeness of a many-fermion state was defined to be its entropy relative tothe unique free state with the same 1-matrix.  In this article, we prove that the nonfreeness of a state is the minimum ofits entropy relative to all free states. We also extend the definition ofnonfreeness to many-boson states and discuss a couple of examples.

Correlation in fermion or boson systems as the minimum of entropy relative to all free states