  This article is a brief review of "nonfreeness" and related measures of
"correlation" for many-fermion systems.
  The many-fermion states we deem "uncorrelated" are the gauge-invariant
quasi-free states. Uncorrelated states of systems of finitely many fermions we
call simply "free" states. Slater determinant states are free; all other free
states are "substates" of Slater determinant states or limits of such.
  The nonfreeness of a many-fermion state equals the minimum of its entropy
relative to all free states. Correlation functionals closely related to
nonfreeness can be defined in terms of R\'enyi entropies; nonfreeness is the
one that uses Shannon entropy. These correlation functionals all share
desirable additivity and monotonicity properties, but nonfreeness has some
additional attractive properties.
