International Workshop on 'New challenges in Reduced Density Matrix Functional Theory: Symmetries, time-evolution and entanglement'
Measures for electron correlation: NonfreenessNorbert Mauser
The relative entropy of a correlated state and an uncorrelated reference state is a reasonable measure for the degree of correlations . A particular measure, coined “nonfreeness” was introduced by Gottlieb and Mauser .
A key question is to 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, so “nonfreenes” is unique and reasonable. Held and Mauser have demonstrated  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.
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 K. Held and N.J. Mauser, European Physical Journal B 86 328 (2013).
 A. Gottlieb and N.J. Mauser, „Nonfreeness and related functionals for measuring correlation in many-fermion states”, arXiv:1510.04573.