1,398 research outputs found
Coupled Electron Ion Monte Carlo Calculations of Dense Metallic Hydrogen
We present a new Monte Carlo method which couples Path Integral for finite
temperature protons with Quantum Monte Carlo for ground state electrons, and we
apply it to metallic hydrogen for pressures beyond molecular dissociation. We
report data for the equation of state for temperatures across the melting of
the proton crystal. Our data exhibit more structure and higher melting
temperatures of the proton crystal than Car-Parrinello Molecular Dynamics
results. This method fills the gap between high temperature electron-proton
Path Integral and ground state Diffusion Monte Carlo methods
Metropolis Methods for Quantum Monte Carlo Simulations
Since its first description fifty years ago, the Metropolis Monte Carlo
method has been used in a variety of different ways for the simulation of
continuum quantum many-body systems. This paper will consider some of the
generalizations of the Metropolis algorithm employed in quantum Monte Carlo:
Variational Monte Carlo, dynamical methods for projector monte carlo ({\it
i.e.} diffusion Monte Carlo with rejection), multilevel sampling in path
integral Monte Carlo, the sampling of permutations, cluster methods for lattice
models, the penalty method for coupled electron-ionic systems and the Bayesian
analysis of imaginary time correlation functions.Comment: Proceedings of "Monte Carlo Methods in the Physical Sciences"
Celebrating the 50th Anniversary of the Metropolis Algorith
Quantum Monte Carlo Simulation of the High-Pressure Molecular-Atomic Crossover in Fluid Hydrogen
A first-order liquid-liquid phase transition in high-pressure hydrogen
between molecular and atomic fluid phases has been predicted in computer
simulations using ab initio molecular dynamics approaches. However, experiments
indicate that molecular dissociation may occur through a continuous crossover
rather than a first-order transition. Here we study the nature of molecular
dissociation in fluid hydrogen using an alternative simulation technique in
which electronic correlation is computed within quantum Monte Carlo, the
so-called Coupled Electron Ion Monte Carlo (CEIMC) method. We find no evidence
for a first-order liquid-liquid phase transition.Comment: 4 pages, 5 figures; content changed; accepted for publication in
Phys. Rev. Let
Lowering of the Kinetic Energy in Interacting Quantum Systems
Interactions never lower the ground state kinetic energy of a quantum system.
However, at nonzero temperature, where the system occupies a thermal
distribution of states, interactions can reduce the kinetic energy below the
noninteracting value. This can be demonstrated from a first order weak coupling
expansion. Simulations (both variational and restricted path integral Monte
Carlo) of the electron gas model and dense hydrogen confirm this and show that
in contrast to the ground state case, at nonzero temperature the population of
low momentum states can be increased relative to the free Fermi distribution.
This effect is not seen in simulations of liquid He-3.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett., June, 200
Path Integral Monte Carlo Simulations for Fermion Systems: Pairing in the Electron-Hole Plasma
We review the path integral method wherein quantum systems are mapped with
Feynman's path integrals onto a classical system of "ring-polymers" and then
simulated with the Monte Carlo technique. Bose or Fermi statistics correspond
to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity
and Bose condensation result from macroscopic exchange of bosons. To map
fermions onto a positive probability distribution, one must restrict the paths
to lie in regions where the fermion density matrix is positive. We discuss a
recent application to the two-component electron-hole plasma. At low
temperature excitons and bi-excitons form. We have used nodal surfaces
incorporating paired fermions and see evidence of a Bose condensation in the
energy, specific heat and superfluid density. In the restricted path integral
picture, pairing appears as intertwined electron-hole paths. Bose condensation
occurs when these intertwined paths wind around the periodic boundaries.Comment: 14 pages, 7 figures Prepared for the 1999 International Conference on
Strongly Coupled Coulomb Systems, Saint-Malo, Franc
Spin-Polarization transition in the two dimensional electron gas
We present a numerical study of magnetic phases of the 2D electron gas near
freezing. The calculations are performed by diffusion Monte Carlo in the fixed
node approximation. At variance with the 3D case we find no evidence for the
stability of a partially polarized phase. With plane wave nodes in the trial
function, the polarization transition takes place at Rs=20, whereas the best
available estimates locate Wigner crystallization around Rs=35. Using an
improved nodal structure, featuring optimized backflow correlations, we confirm
the existence of a stability range for the polarized phase, although somewhat
shrunk, at densities achievable nowadays in 2 dimensional hole gases in
semiconductor heterostructures . The spin susceptibility of the unpolarized
phase at the magnetic transition is approximately 30 times the Pauli
susceptibility.Comment: 7 pages, 4 figure
Many-body wavefunctions for normal liquid He
We present new trial wave-functions which include 3-body correlations into
the backflow coordinates and a 4-body symmetric potential. We show that our
wavefunctions lower the energy enough to stabilize the ground state energies of
normal liquid He in the unpolarized state at all pressures in agreement
with experiment; however, quantitative discrepancies remain. Further, we
include strong spin coupling into the Fermi liquid by adapting pairing wave
functions. We demonstrate a new, numerically stable method to evaluate pairing
functions which is also useful for Path Integrals calculations at low, but
non-zero temperatures.Comment: 5 page
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