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Progress of Theoretical Physics

General Theorem for Interacting Many-Particle Systems

節夫 三沢

Published Jun 1, 1968• 5 citations
Publisher

Abstract

It is shown that the kinetic energy of any interacting many-particle system in its ground state is a monotone increasing function with respect to the absolute value of the coupling constant (or the interaction strength). A concise derivation of the virial theorem for simple power law potentials is also presented with particular emphasis on the case of an electron gas in a uniform background of positive charge.

DOI
10.1143/ptp.39.1426

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It is shown that the kinetic energy of any interacting many-particle system in its ground state is a monotone increasing function with respect to the absolute value of the coupling constant (or the interaction strength). A concise derivation of the virial theorem for simple power law potentials is also presented with particular emphasis on the case of an electron gas in a uniform background of positive charge.
It is shown by dimensional analysis that Pauli's ground-state-energy theorem, which expresses the ground-state energy in the form of a coupling-constant integration of the potential energy, and the virial theorem are equivalent for a many-particle system interacting with a homogeneous interparticle potential ($\ensuremath{\propto}{r}^{l}$) in arbitrary (ddd) dimensions. The finite-temperature case is also discussed.
Starting from the stationary Schrödinger equation for a system of identical interacting particles, the three-dimensional differential force law (DFL) is derived in terms of the kinetic energy density tensor with components tαβ(x), the particle density n(x), and the potential. The most general vector field h(x) is given such that integrating the scalar product of h with the DFL over an arbitrary volume Ω yields theorems involving in their volume integrals the tensor components only in the form t≡∑3α=1tαα (if at all) t being the positive definite density of kinetic energy. The procedure results in four integral theorems: (i) balance equation of forces, (ii) balance equation of torques, (iii) the generalized virial theorem, and (iv) a new exact theorem which can be regarded as vector theorem on the first moment of the kinetic energy density. The new theorem is shown to imply validity of the other three, and therefore is more comprehensive than they.
When the kinetic energy density in k (defined positive definite) of a system of one-dimensional, non-interacting fermions is approximated by an ordinary function of the density rho , and of the lowest n derivatives of rho , in k >or=0 can satisfy the differential virial theorem for arbitrary density distributions only if in k actually depends on rho and rho ' only. Thus the case n>1 is ruled out, and one is left with the result in k = kappa rho 3 + rho ' 2 /(8 rho ) ( kappa >or=0) already obtained in previous work.
The differential virial theorem of March and Young for N fermions moving in a common one-dimensional potential energy V(x) is here combined with the Euler equation of density functional theory expressing the constancy of the chemical potential throughout the entire inhomogeneous particle density. The functional derivative of the single-particle kinetic energy is thereby expressed directly in terms of the kinetic energy density; a line integral being involved in establishing the connection. This result is then used to establish a formula measuring departures from simple homogeneity of the kinetic energy functional: a matter of current interest in density functional theory. Finally, the general theory of the functional derivative of the single-particle kinetic energy with respect to the particle density is exemplified for the case of harmonic confinement of fermions in one dimension.
The virial theorem is used to derive an exact hierarchy of coupled equations for the kinetic energy functional T[\ensuremath{\rho}] of an inhomogeneous system of noninteracting fermions. The hierarchy links the nth functional derivative of T[\ensuremath{\rho}] with respect to the particle density \ensuremath{\rho}(r) to the (n+1)th functional derivative and to \ensuremath{\rho}(r) for n=1,2,. . . . These functional derivatives are the analogs of the so-called ``direct correlation functions'' in the theory of classical fluids, and the hierarchy obtained resembles the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for the distribution functions associated with such fluids. Some general properties of the two- and three-point correlation functions are described, in particular, their relations to the linear and quadratic response functions that describe the response of the inhomogeneous system to a change in the potential. The hierarchy is then used to obtain several equivalent expressions for T[\ensuremath{\rho}], one of which is an infinite series of successively higher-order terms, of which the nth term represents the contribution from the n-point correlation function and the densities associated with those points. The equations are then examined in the homogeneous (uniform) gas limit and the first and second equations are verified explicitly with the aid of the associated response functions. The analysis indicates that the infinite series expression converges rapidly for the three-dimensional gas; it also reveals that for both the one-dimensional and two-dimensional gas, the series terminates after the first few terms, suggesting that the structure of T[\ensuremath{\rho}] is much simpler in one and two dimensions than in three dimensions.