Max Planck Inst Phys Komplexer Syst Bayreuther Str 40 Haus 16 D-01187 Dresden Germany
Max Planck Inst Festkorperforsch D-70569 Stuttgart Germany
Tech Univ Dresden Inst Theoret Phys D-01062 Dresden Germany
Univ Neuchatel Inst Phys Ch-2000 Neuchatel Switzerland

Exactly solvable models (the two-electron two-site Hubbard model and the Bardeen-Cooper-Schrieffer model) are revisited in terms of their natural occupation numbers (eigenvalues of the true, i.e., correlated, one-particle density matrix). Entropylike expressions that may serve as measures of the correlation strength are related to possible reference states for the definition of correlation and discussed as functions of the system parameters.
 

Physical Review B, 55 10270-10277, 1997.