3,303 research outputs found
A diffusion Monte Carlo algorithm with very small time‐step errors
We propose modifications to the simple diffusion Monte Carlo algorithm that greatly reduce the time‐step error. The improved algorithm has a time‐step error smaller by a factor of 70 to 300 in the energy of Be, Li2 and Ne. For other observables the improvement is yet larger. The effective time step possible with the improved algorithm is typically a factor of a few hundred larger than the time step used in domain Green function Monte Carlo. We also present an optimized 109 parameter trial wave function for Be which, used in combination with our algorithm, yields an exceedingly accurate ground state energy. A simple solution to the population control bias in diffusion Monte Carlo is also discussed
Quantum simulations of the superfluid-insulator transition for two-dimensional, disordered, hard-core bosons
We introduce two novel quantum Monte Carlo methods and employ them to study
the superfluid-insulator transition in a two-dimensional system of hard-core
bosons. One of the methods is appropriate for zero temperature and is based
upon Green's function Monte Carlo; the other is a finite-temperature world-line
cluster algorithm. In each case we find that the dynamical exponent is
consistent with the theoretical prediction of by Fisher and co-workers.Comment: Revtex, 10 pages, 3 figures (postscript files attached at end,
separated by %%%%%% Fig # %%%%%, where # is 1-3). LA-UR-94-270
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
Green Function Monte Carlo with Stochastic Reconfiguration
A new method for the stabilization of the sign problem in the Green Function
Monte Carlo technique is proposed. The method is devised for real lattice
Hamiltonians and is based on an iterative ''stochastic reconfiguration'' scheme
which introduces some bias but allows a stable simulation with constant sign.
The systematic reduction of this bias is in principle possible. The method is
applied to the frustrated J1-J2 Heisenberg model, and tested against exact
diagonalization data. Evidence of a finite spin gap for J2/J1 >~ 0.4 is found
in the thermodynamic limit.Comment: 13 pages, RevTeX + 3 encapsulated postscript figure
Radiation studies for GaAs in the ATLAS Inner Detector
We estimate the hardness factors and the equivalent 1 MeV neutron fluences
for hadrons fluences expected at the GaAs positions wheels in the ATLAS Inner
Detector. On this basis the degradation of the GaAs particle detectors made
from different substrates as a function of years LHC operation is predicted.Comment: 11 pages, 6 Postscript figures, uses elsart.cls, submitted to Nucl.
Inst. and Met
Ground state parameters, finite-size scaling, and low-temperature properties of the two-dimensional S=1/2 XY model
We present high-precision quantum Monte Carlo results for the S=1/2 XY model
on a two-dimensional square lattice, in the ground state as well as at finite
temperature. The energy, the spin stiffness, the magnetization, and the
susceptibility are calculated and extrapolated to the thermodynamic limit. For
the ground state, we test a variety of finite-size scaling predictions of
effective Lagrangian theory and find good agreement and consistency between the
finite-size corrections for different quantities. The low-temperature behavior
of the susceptibility and the internal energy is also in good agreement with
theoretical predictions.Comment: 6 pages, 8 figure
Size-dependent permittivity and intrinsic optical anisotropy of nanometric gold thin films: A density functional theory study
Physical properties of materials are known to be different from the bulk at the nanometer scale. In this context, the dependence of optical properties of nanometric gold thin films with respect to film thickness is studied using density functional theory (DFT). We find that the in-plane plasma frequency of the gold thin film decreases with decreasing thickness and that the optical permittivity tensor is highly anisotropic as well as thickness dependent. Quantitative knowledge of planar metal film permittivity's thickness dependence can improve the accuracy and reliability of the designs of plasmonic devices and electromagnetic metamaterials. The strong anisotropy observed may become an alternative method of realizing indefinite media. © 2013 Optical Society of America
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