The momentum distribution of the unpolarized uniform electron gas in its Fermi-liquid regime, n(k, rs), with the momenta k measured in units of the Fermi wave number kF and with the density parameter rs, is constructed with the help of the convex Kulik function G(x). It is assumed that n(0, rs), n(1 ± , rs), the on-top pair density g(0, rs) and the kinetic energy t(rs) are known (respectively, from accurate calculations for rs = 1, ..., 5, from the solution of the Overhauser model, and from Quantum Monte Carlo calculations via the virial theorem). Information from the high-and the low-density limit, corresponding to the random-phase approximation and to the Wigner crystal limit, is used. The result is an accurate parametrization of n(k, rs), which fulfills most of the known exact constraints. It is in agreement with the effective-potential calculations of Takada and Yasuhara [Phys. Rev. B 44, 7879 (1991)], is compatible with Quantum Monte Carlo data, and is valid in the density range rs 12. The corresponding cumulant expansions of the pair density and of the static structure factor are discussed, and some exact limits are derived.

-A generalized density functional theory (DFT) is proposed based on a generalized Hohenberg-Kohn theorem with the pair density as the key quantity and the kinetic energy as a universal functional of the pair density. It is assumed that there exists an effective interaction potential which, via the corresponding two-particle (2P) Schrodinger equation, generates 2P orbitals (geminals) from which follows the pair density, just as in the conventional DFT the density follows from 1P orbitals (as the solutions of an effective 1P Schrodinger equation). According to three different representations (natural spectral resolutions) of the pair density or the cumulant pair densities in terms of geminals, three versions of a pair DFT (PDFT) are formally sketched. Also considered are the relation between electron correlation and particle-number fluctuations in fragments of the system and the use of the pair density for an estimation of such fluctuations. 0 1996 John Wiley & Sons, Inc.

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-the v-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. 0 1995 John Wiley & Sons, Inc

For electronic systems, a simple property of the recently introduced kinetic energy T as a functional of the pair density n(r1,r2)is derived. Approximate explicit expressions for T[n] are presented.

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The recently developed concept of a correlation entropy, S, as a quantitative measure of the correlation strength present in a correlated quantum many-body state is applied to the ground states of the He isoelectronic series He(Z) with varying nuclear charge Z and of the Hooke’s law model HLM(ω) with varying oscillator frequency ω. S is constructed from the natural orbital occupation numbers. It vanishes for weak correlation (large coupling constants Z or ω), and increases monotonically with decreasing Z or ω (strengthening correlation). A reduced correlation energy per particle Δecorr and a dimensionless ratio ε=|Ecorr/E| are introduced which vanish asymptotically in the weak correlation limit in contrast to Ecorr and ecorr=Ecorr/N. These two intensive quantities, Δecorr and ε, are compared with s=S/N. For both model systems, dΔecorr/ds⩾0 and dε/ds⩾0 (which modifies Collins’ conjecture that |Ecorr|∼S).