Properties of nonfreeness: An entropy measure of electron correlation
The "nonfreeness" of a many-fermion state omega is the entropy of omega relative to the free state that has the same 1-particle statistics as omega. The nonfreeness of a pure state is the same as its "particle-hole symmetric correlation entropy", a variant of an established measure of electron correlation. However, nonfreeness is also defined for mixed states, and this allows one to compare the nonfreeness of subsystems to the nonfreeness of the whole. Nonfreeness of a part does not exceed that in the whole; nonfreeness is additive over independent subsystems; and nonfreeness is superadditive over subsystems that are independent on the 1-particle level.
correlation entropy fermion correlation relative entropy fermion fock space density operator quasifree entropy