  The relative entropy of a correlated state and an uncorrelated reference
state is a reasonable measure for the degree of correlations. A key question is
however which uncorrelated state to compare to. The relative entropy becomes
minimal for the uncorrelated reference state that has the same one-particle
density matrix as the correlated state. Hence, this particular measure, coined
nonfreeness, is unique and reasonable. We demonstrate that for relevant
physical situations, such as finite temperatures or a correlation enhanced
orbital splitting, other choices of the uncorrelated state, even educated
guesses, overestimate correlations.
