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Physical Review Letters

Thermodynamic perturbation scheme for the ground-state energy of Lennard-Jones bosons

M. A. Sol̆s, Valdir C. Aguilera-Navarro, MANUEL DE LLANO, R. Guardiola

Published Nov 16, 1987• 13 citations
Publisher

Abstract

The thermodynamic perturbation scheme so successful for classical-liquid equations of state is applied to the ground-state energy of a many-boson system interacting pairwise via the Lennard-Jones intermolecular potential. The ensuing density and attractive-coupling double expansion for the energy is series analyzed with Pad\'e techniques, as well as generalizations thereof. Results through sixth order compare with Green's-function Monte Carlo simulations at the 1% level of accuracy.

DOI
10.1103/physrevlett.59.2322

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The thermodynamic perturbation scheme so successful for classical-liquid equations of state is applied to the ground-state energy of a many-boson system interacting pairwise via the Lennard-Jones intermolecular potential. The ensuing density and attractive-coupling double expansion for the energy is series analyzed with Pad'e techniques, as well as generalizations thereof. Results through sixth order compare with Green's-function Monte Carlo simulations at the 1% level of accuracy.
The thermodynamic perturbation scheme for the very successful construction of classical liquid equations of state is applied in the quantum regime to the ground-state energy of a many-boson system interacting pairwise via a Lennard-Jones inter­ molecular potential. The ensuing density and attractive-coupling double expansion for the energy is series-analyzed with Pade techniques, as well as generalizations thereof. Results through sixth order compare with the Green Function Monte Carlo simula­ tions for the same system at the one-percent level of accuracy.
The exact, low-density many-boson expansion for the ground-state energy is rearranged in a quantum-thermodynamic perturbation series. Extrapolation to physical densities is implemented order by order via Pad\'e-type approximants. Several ways of splitting the pair Lennard-Jones (LJ) or Aziz potentials are considered, including the Barker-Henderson (BH) and Weeks-Chandler-Andersen (WCA) schemes familiar from classical fluid studies. Results obtained using the LJ interaction with WCA splitting, when compared with benchmark simulation data, agree excellently in energy and density, but only moderately so in sound velocity. For the Aziz potential, however---perhaps because of a softer core and/or its ability to support a two-particle bound state---, neither potential splitting (BH or WCA) gave results of the LJ quality, but a method intermediate between BH and WCA proves to be superior. Our results are also compared with other recent alternative calculations, both variational and perturbative.
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A more powerful version of a numerical method published before is given which permits the integration of the Schr\"odinger equation for a large ensemble of particles. A new method for obtaining quantum expectation values is outlined. Numerical results are tabulated for the energy of the ground state of a system of 32 bosons interacting by Lennard-Jones potentials and with periodic boundary conditions. This is a commonly used model of liquid He4{\mathrm{He}}^{4}He4. The present results are substantially lower and closer to experiment than variational estimates based on a Jastrow-type wave function.
The London ground-state energy formula as a function of number density for a system of identical boson hard spheres, corrected for the reduced mass of a pair of particles in a "sphere-of-influence" picture, and generalized to fermion hard-sphere systems with two and four intrinsic degrees of freedom, has a double-pole at the ultimate regular (or periodic, e.g., face-centered-cubic) close-packing density usually associated with a crystalline branch. Improved fluid branches are constructed based upon exact, field-theoretic perturbation-theory low-density expansions for many-boson and many-fermion systems, extrapolated to intermediate densities via Padé and other approximants, but whose ultimate density is irregular or random closest close-packing as suggested in studies of a classical system of hard spheres. Results show substantially improved agreement with the best available Green-function Monte Carlo and diffusion Monte Carlo simulations for bosons, as well as with ladder, variational Fermi hypernetted chain, and so-called L -expansion data for two-component fermions.
In the interest of polishing the parquet approach to microscopic calculations of ground-state properties of zero-temperature quantum fluids, we have developed a perturbative scheme for the systematic improvement of the four-point function which retains the correct behavior of the parquet diagrams in both the small-r and small-k limits. Numerical implementation of this scheme indicates significantly improved agreement with binding energies calculated with Green's-function Monte Carlo techniques (i.e., differences in liquid 4He^{4}\mathrm{He}4He are less than 1.5%). The charged Bose gas and the hard-sphere system are also considered. Remaining difficulties in the calculations of the liquid structure function are noted and discussed.