Correlation strength and information entropy

The correlation present in the nondegenerate ground state of an interacting Fermi system is discussed in terms of reduced density matrices and their cumulant expansion. By generalizing a result obtained for the interacting uniform electron gas (correlation induced exchange‐hole narrowing), possible measures of the correlation strength in terms of natural occupation numbers (the eigenvalues of the true one‐particle density matrix) are introduced. These quantities‐thev‐order nonidempotency and the information entropy of the natural occupation numbers‐result from the correlated many‐body wave function and characterize the ground‐state correlation in addition to the usual correlation energy. The uniform electron gas serves as a first illustrative example.