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Physical Review A

Strong-correlation limit of four electrons in an isotropic harmonic trap

Jerzy Ciosłowski, Ewa Grzebielucha

Published Mar 14, 2008• 25 citations
Publisher

Abstract

Asymptotically exact wave functions of the four-electron harmonium atom at the strong-correlation limit are given by Slater determinants multiplied by a Jastrow factor in the form of a single exponential function with the argument bilinear in the interelectron distances. The spin orbitals that enter the determinants set the multiplicities and angular momenta of individual states whereas the Jastrow factor fully accounts for the electron correlation. Asymptotically exact expressions for the corresponding electron densities, which can be computed from such wave functions with the help of the saddle-point approximation, have the form of products of Gaussian functions and polynomials in the Cartesian coordinates.

DOI
10.1103/physreva.77.032508

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•Jan 6, 2015•3 citations
Asymptotically exact wave functions of the four-electron harmonium atom at the strong-correlation limit are given by Slater determinants multiplied by a Jastrow factor in the form of a single exponential function with the argument bilinear in the interelectron distances. The spin orbitals that enter the determinants set the multiplicities and angular momenta of individual states whereas the Jastrow factor fully accounts for the electron correlation. Asymptotically exact expressions for the corresponding electron densities, which can be computed from such wave functions with the help of the saddle-point approximation, have the form of products of Gaussian functions and polynomials in the Cartesian coordinates.
Explicit asymptotic expressions for natural orbitals and their occupancies are derived for the harmonium atom at the strong-correlation limit at which the confinement strength omega tends to zero. Unlike in systems with moderate correlation effects, the occupancies at the omega-->0 limit (derived from occupation amplitudes with alternating sign patterns) are vanishingly small and asymptotically independent of the angular momentum, forming a geometric progression with the scale factor proportional to omega(1/3) and the common ratio of ca. 0.0186. The radial components of the natural orbitals are given by products of polynomials and Gaussian functions that, as expected, peak at approximately half of the equilibrium interelectron distance.
Explicit asymptotic expressions are derived for the reduced one-electron density matrices (the 1-matrices) of strongly correlated two- and three-electron harmonium atoms in the ground and first excited states. These expressions, which are valid at the limit of small confinement strength ω, yield electron densities and kinetic energies in agreement with the published values. In addition, they reveal the ω(5/6) asymptotic scaling of the exchange components of the electron-electron repulsion energies that differs from the ω(2/3) scaling of their Coulomb and correlation counterparts. The natural orbitals of the totally symmetric ground state of the two-electron harmonium atom are found to possess collective occupancies that follow a mixed power/Gaussian dependence on the angular momentum in variance with the simple power-law prediction of Hill's asymptotics. Providing rigorous constraints on energies as functionals of 1-matrices, these results are expected to facilitate development of approximate implementations of the density matrix functional theory and ensure their proper description of strongly correlated systems.
In the harmonium atom, the collective occupancies {n(l)} of natural orbitals with different angular momenta l can be rigorously studied for those values of the confinement strength omega that lead to ground-state wave functions with correlation factors given by polynomials of kth order. Analysis of two such cases, with omega equal to 12 (k=1) and 110 (k=2), uncovers some interesting properties of {n(l)}, which turn out to be given by rational functions of pi(12) with simple coefficients. In particular, contrary to naive expectations, the collective occupancies for l>2 actually decrease upon strengthening of correlation. The explicit formulas for l-matrices and {n(l)} can be readily utilized in testing, calibration, and benchmarking of approximate implementations of the density matrix functional theory (DMFT).
The physical origin of the feature of strong correlation between electrons is mainly ascribed to doubly occupied atomic orbitals. Two-particle wave functions are believed necessary to describe strongly correlated systems. We emphasize that the on-site Coulomb interaction, which is important for strong correlation, should appear in equations as a natural term. Hartree-Fock-like self-consistent equations are derived with wave functions including two-particle wave functions for such systems. In the self-consistent equations, a strong correlation term is separated from the mean field. To our knowledge, this is the first attempt to formulate self-consistent equations in which the strong correlation effect is directly embodied. Some features of the equations are discussed. Koopmans' theorem and Brillouin's theorem are applied to the equations. The former is valid for singly occupied orbitals, and the latter is entirely available. By separating an angular correlation factor which is believed important for a strong correlation effect, an approximate form of a two-particle wave function is suggested. This kind of wave function causes the six multiple integrals to degrade to triple integrals. Under such forms of two-particle wave functions, corresponding Roothaan equations are established. Through these steps, it is believed that the equations become solvable within present computational limits.
Standard approaches to approximating solutions to the Schroedinger equation rely upon the assumption that there is a single dominant electronic configuration with a small number of low-lying excited states. As such, they are referred to as single reference approaches and are accurate only when the band gap is small in comparison with the inter-electronic repulsion of the valence electrons. Systems for which this is the case are weakly-correlated. If the inter-electronic repulsion larger than the band gap, then a single electronic reference is not an appropriate ansatz for the physical wavefunction. As the strength of the interaction is increased many Slater determinants become near-degenerate, each providing non-negligible contributions. Such systems are strongly-correlated. The goal of the thesis was to employ wavefunctions which are: non-trivial, tractable, size-consistent and extensive, and not given in an active space. We have outlined approaches based on wavefunction forms more suited to describe strong correlation. Built on the Lie algebra su(2), our first trial wavefunction was the Bethe ansatz solution to the reduced BCS Hamiltonian derived by Richardson. This wavefunction is expressed as a product of quasi-particles each parametrized in terms of quasi-momenta. Consistency among the quasi-momenta is ensured by a set of non-linear equations to be solved numerically. This approach is feasible in both variational and projected flavours. We next considered a wavefunction of the same form without requiring it to be an eigenvector of the reduced BCS Hamiltonian. A variational approach in this case is only feasible provided there are two equivalent descriptions in terms of particles and holes. In any case, a bivariational principle and a projected approach are both feasible. Also based on su(2), there is AP1roG (coupled cluster pair doubles). Open-shell singlets and triplets were constructed with the Lie algebras sp(N) and so(4N). The resulting formulae appear to be infeasible unless it is possible to efficiently compute generalizations of determinants to three-dimensional objects. Treating each spatial orbital individually, all four possible occupations close the Lie superalgebra gl(2|2).