In a previous work (Ghiringhelli L. M. and Delle Site L., Phys. Rev. B , 77 (2008) 073104) we have proposed a methodological approach to determine the electron correlations contained in the kinetic functional. Its application to the test case of an almost uniform interacting electron gas led to a local density functional of the form ∫ρ( r )lnρ( r )d r . In this work we discuss the interpretation of such a result by performing an analysis of the formal and numerical properties of the functional under the process of coordinates' scaling. On the basis of such an analysis we propose a local analytic form of the kinetic functional of which the previous numerical result is a first-order approximation.
In a previous work (Ghiringhelli L. M. and Delle Site L., Phys. Rev. B , 77 (2008) 073104) we have proposed a methodological approach to determine the electron correlations contained in the kinetic functional. Its application to the test case of an almost uniform interacting electron gas led to a local density functional of the form ∫ρ( r )lnρ( r )d r . In this work we discuss the interpretation of such a result by performing an analysis of the formal and numerical properties of the functional under the process of coordinates' scaling. On the basis of such an analysis we propose a local analytic form of the kinetic functional of which the previous numerical result is a first-order approximation.
In a previous work [L. Delle Site, J. Phys. A: Math. Theor. 40, 2787 (2007)], the derivation of an analytic expression for the kinetic functional of a many-body electron system has been proposed. Though analytical, the formula is still nonlocal (multidimensional) and thus not ideal for numerical applications. In this work, by treating the test case of a uniform gas of interacting spinless electrons, we propose a computational protocol which combines the previous analytic results with the Monte Carlo sampling of electronic configurations in space. This, we show, leads to an internally consistent scheme to design well founded local kinetic functionals.
Within density functional theory, a coordinate-scaling relation for the coupling-constant dependence of the exchange-correlation kernel ${f}_{\mathrm{xc}}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}};\ensuremath{\omega})$ is utilized to express the correlation energy of a many-electron system in terms of fxc. As a test of several of the available approximations for the exchange-correlation kernel, or equivalently the local-field factor, we calculate the uniform-gas correlation energy. While the random phase approximation (fxc= 0) makes the correlation energy per electron too negative by about 0.5 eV, the adiabatic local-density approximation [fxc=fxc(q= $0,\ensuremath{\omega}$ = 0)] makes a comparable error in the opposite direction. The adiabatic nonlocal approximation [fxc= ${f}_{\mathrm{xc}}(q,\ensuremath{\omega}$ = 0)] reduces this error to about 0.1 eV, and inclusion of the full frequency dependence [fxc= ${f}_{\mathrm{xc}}(q,\ensuremath{\omega})]$ in an approximate parametrization reduces it further to less than 0.02 eV. We also report the wave-vector analysis and the imaginary-frequency analysis of the correlation energy for each choice of kernel.
We consider a nearly (or quasi) uniform gas of interacting electrons for which spin statistics play a crucial role. A previously developed procedure, based on the extension of the Levy-Lieb constrained search principle and Monte Carlo sampling of electron configurations in space, allows us to approximate the form of the kinetic-energy functional. For a spinless electron gas, this procedure led to a correlation term, which had the form of the Shannon entropy, but the resulting kinetic-energy functional does not satisfy the Lieb-Thirring inequality, which is rigorous and one of the most general relations regarding the kinetic energy. In this paper, we show that when the fermionic character of the electrons is included via a statistical spin approach, our procedure leads to correlation terms, which also have the form of the Shannon entropy and the resulting kinetic-energy functional does satisfy the Lieb-Thirring inequality. In this way we further strengthen the connection between Shannon entropy and electron correlation and, more generally, between information theory and quantum mechanics.
The Wigner regime of a system of electrons in an external field is characterized by a low electron density and a high electron-interaction energy relative to the kinetic energy. The low-correlation regime is in turn described by a high electron density and an electron-interaction energy smaller than the kinetic energy. The Wigner regime of a nonuniform-electron-density system is investigated via quantal density functional theory (QDFT). Within QDFT, the contributions of electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects are separately delineated and explicitly defined. The nonuniform-electron-density system studied is that of the Hooke's atom in the Wigner regime, for which the exact wave function is derived. As such, the results of the QDFT analysis are exact. It is observed that in comparison to the low-correlation case, not only is the electron-interaction energy greater than the kinetic energy as a fraction of the total energy, but so are its individual Hartree, Pauli, and Coulomb components. The ionization potential as a fraction of the total energy too is greater. But most significantly, in the Wigner regime, the correlation-kinetic energy as a fraction of both the electron-interaction and the total energy is substantially greater than in the low-correlation case. Hence, we propose that the Wigner regime now also be characterized by a high correlation-kinetic energy. The kinetic energy as a fraction of the total energy, however, is less than in the low-correlation case. This fact and the high correlation-kinetic energy value in the Wigner regime is explained by the concept of “quantal compression” of the kinetic energy density derived from QDFT. The corresponding results for the low-correlation case are in turn a consequence of a “quantal decompression” of the kinetic energy density. From the QDFT analysis, the exact values for the Kohn-Sham theory “exchange-correlation” and “correlation” energy functionals of the density and their respective functional derivatives are also obtained. These results ought to be of value in the construction and testing of approximate energy functionals valid for the Wigner regime.
In this paper, we have presented an analytical model of two electron systems consisting of a many particle correlated wave function with some variational parameters α, λ and µ and used it to quantify the electron-electron correlation described by the wave function containing explicitly r12 (inter atomic distance between two electrons) dependent term. The single particle wave functions and the charge densities have been extracted from the said correlated wave function both for the uncorrelated and correlated systems in coordinate space and its momentum analogs have been obtained by taking the Fourier transform of the coordinate analogs. We have computed and presented the results of the numerical values of the theoretic information entropies of the Shannon entropy, Fisher information entropy, Shannon power and the FisherShannon product. The numerical values are consistently found to satisfy the Beckner, Bialynicki-Birula and Mycielski (BBM ) inequality relation; Stam-Cramer-Rao inequalities or Fisher based uncertainty relation and Fisher-Shannon product relation for the uncorrelated and correlated systems in both the coordinate and momentum spaces.
Accurate treatment of the electronic correlation in inhomogeneous electronic systems, combined with the ability to capture the correlation energy of the homogeneous electron gas, allows to reach high predictive power in the application of density-functional theory. For two-dimensional systems we can achieve this goal by generalizing our previous approximation [Phys. Rev. B 79, 085316 (2009)] to a parameter-free form, which reproduces the correlation energy of the homogeneous gas while preserving the ability to deal with inhomogeneous systems. The resulting functional is shown to be very accurate for finite systems with an arbitrary number of electrons with respect to numerically exact reference data.