CORRELATION STRENGTH AND INFORMATION ENTROPY

The correlation present in the nondegenerate ground state of an intera cting Fermi system is discussed in terms of reduced density matrices a nd their cumulant expansion. By generalizing a result obtained for the interacting uniform electron gas (correlation induced exchange-hole n arrowing), possible measures of the correlation strength in terms of n atural occupation numbers (the eigenvalues of the true one-particle de nsity matrix) are introduced. These quantities-the nu-order nonidempot ency and the information entropy of the natural occupation numbers-res ult from the correlated many-body wave function and characterize the g round-state correlation in addition to the usual correlation energy. T he uniform electron gas serves as a first illustrative example. (C) 19 95 John Wiley & Sons, Inc.