International Workshop on 'New challenges in Reduced Density Matrix Functional Theory: Symmetries, time-evolution and entanglement'
Measures for electron correlation: NonfreenessNorbert Mauser
Wolfgang Pauli Institute, Austria
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.
 A. Gottlieb and N.J. Mauser, Phys. Rev. Lett. 95 213 (2005).
 A. Gottlieb and N.J. Mauser, Int. J. of Quantum Information 5 10 (2007).
 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.