1,068 research outputs found
Open-source development experiences in scientific software: the HANDE quantum Monte Carlo project
The HANDE quantum Monte Carlo project offers accessible stochastic algorithms
for general use for scientists in the field of quantum chemistry. HANDE is an
ambitious and general high-performance code developed by a
geographically-dispersed team with a variety of backgrounds in computational
science. In the course of preparing a public, open-source release, we have
taken this opportunity to step back and look at what we have done and what we
hope to do in the future. We pay particular attention to development processes,
the approach taken to train students joining the project, and how a flat
hierarchical structure aids communicationComment: 6 pages. Submission to WSSSPE
Accurate exchange-correlation energies for the warm dense electron gas
Density matrix quantum Monte Carlo (DMQMC) is used to sample exact-on-average
-body density matrices for uniform electron gas systems of up to 10
matrix elements via a stochastic solution of the Bloch equation. The results of
these calculations resolve a current debate over the accuracy of the data used
to parametrize finite-temperature density functionals. Exchange-correlation
energies calculated using the real-space restricted path-integral formalism and
the -space configuration path-integral formalism disagree by up to
\% at certain reduced temperatures and densities . Our calculations confirm the accuracy of the configuration
path-integral Monte Carlo results available at high density and bridge the gap
to lower densities, providing trustworthy data in the regime typical of
planetary interiors and solids subject to laser irradiation. We demonstrate
that DMQMC can calculate free energies directly and present exact free energies
for and .Comment: Accepted version: added free energy data and restructured text. Now
includes supplementary materia
Preconditioning and perturbative estimators in full configuration interaction quantum Monte Carlo
We propose the use of preconditioning in FCIQMC which, in combination with
perturbative estimators, greatly increases the efficiency of the algorithm. The
use of preconditioning allows a time step close to unity to be used (without
time-step errors), provided that multiple spawning attempts are made per
walker. We show that this approach substantially reduces statistical noise on
perturbative corrections to initiator error, which improve the accuracy of
FCIQMC but which can suffer from significant noise in the original scheme.
Therefore, the use of preconditioning and perturbatively-corrected estimators
in combination leads to a significantly more efficient algorithm. In addition,
a simpler approach to sampling variational and perturbative estimators in
FCIQMC is presented, which also allows the variance of the energy to be
calculated. These developments are investigated and applied to benzene
(30e,108o), an example where accurate treatment is not possible with the
original method.Comment: 15 pages, 7 figure
The sign problem and population dynamics in the full configuration interaction quantum Monte Carlo method
The recently proposed full configuration interaction quantum Monte Carlo
method allows access to essentially exact ground-state energies of systems of
interacting fermions substantially larger than previously tractable without
knowledge of the nodal structure of the ground-state wave function. We
investigate the nature of the sign problem in this method and how its severity
depends on the system studied. We explain how cancelation of the positive and
negative particles sampling the wave function ensures convergence to a
stochastic representation of the many-fermion ground state and accounts for the
characteristic population dynamics observed in simulations.Comment: 11 pages. 6 figure
Reconstruction of three-dimensional porous media using generative adversarial neural networks
To evaluate the variability of multi-phase flow properties of porous media at
the pore scale, it is necessary to acquire a number of representative samples
of the void-solid structure. While modern x-ray computer tomography has made it
possible to extract three-dimensional images of the pore space, assessment of
the variability in the inherent material properties is often experimentally not
feasible. We present a novel method to reconstruct the solid-void structure of
porous media by applying a generative neural network that allows an implicit
description of the probability distribution represented by three-dimensional
image datasets. We show, by using an adversarial learning approach for neural
networks, that this method of unsupervised learning is able to generate
representative samples of porous media that honor their statistics. We
successfully compare measures of pore morphology, such as the Euler
characteristic, two-point statistics and directional single-phase permeability
of synthetic realizations with the calculated properties of a bead pack, Berea
sandstone, and Ketton limestone. Results show that GANs can be used to
reconstruct high-resolution three-dimensional images of porous media at
different scales that are representative of the morphology of the images used
to train the neural network. The fully convolutional nature of the trained
neural network allows the generation of large samples while maintaining
computational efficiency. Compared to classical stochastic methods of image
reconstruction, the implicit representation of the learned data distribution
can be stored and reused to generate multiple realizations of the pore
structure very rapidly.Comment: 21 pages, 20 figure
The Isolation of a New S-Methyl Benzothioate Compound from a Marine-Derived Streptomyces sp.
The application of an HPLC bioactivity profiling/microtiter plate technique in conjunction with microprobe NMR instrumentation and access to the AntiMarin database has led to the isolation of a new 1. In this example, 1 was isolated from a cytotoxic fraction of an extract obtained from marine-derived Streptomyces sp. cultured on Starch Casein Agar (SCA) medium. The 1D and 2D 1H NMR and ESIMS data obtained from 20 μg of compound 1 fully defined the structure. The known 2 was also isolated and readily dereplicated using this approach
Discovery of a White Dwarf Companion to HD 159062
We report on the discovery of a white dwarf companion to the nearby late G
dwarf star, HD 159062. The companion is detected in 14 years of precise radial
velocity (RV) data, and in high-resolution imaging observations. RVs of HD
159062 from 2003-2018 reveal an acceleration of ,
indicating that it hosts a companion with a long-period orbit. Subsequent
imaging observations with the ShaneAO system on the Lick Observatory 3-meter
Shane telescope, the PHARO AO system on the Palomar Observatory 5-meter
telescope, and the NIRC2 AO system at the Keck II 10-meter telescope reveal a
faint companion 2.7'' from the primary star. We performed relative photometry,
finding magnitudes,
magnitudes, and magnitudes for the companion from
these observations. Analysis of the radial velocities, astrometry, and
photometry reveals that the combined data set can only be reconciled for the
scenario where HD 159062 B is a white dwarf. A full Bayesian analysis of the RV
and imaging data to obtain the cooling age, mass, and orbital parameters of the
white dwarf indicates that the companion is an old white dwarf with an orbital period of years, and a cooling age of Gyr.Comment: 10 pages, 9 figure
Nonequilibrium dynamics of fully frustrated Ising models at T=0
We consider two fully frustrated Ising models: the antiferromagnetic
triangular model in a field of strength, , as well as the Villain
model on the square lattice. After a quench from a disordered initial state to
T=0 we study the nonequilibrium dynamics of both models by Monte Carlo
simulations. In a finite system of linear size, , we define and measure
sample dependent "first passage time", , which is the number of Monte
Carlo steps until the energy is relaxed to the ground-state value. The
distribution of , in particular its mean value, , is shown to
obey the scaling relation, , for both models.
Scaling of the autocorrelation function of the antiferromagnetic triangular
model is shown to involve logarithmic corrections, both at H=0 and at the
field-induced Kosterlitz-Thouless transition, however the autocorrelation
exponent is found to be dependent.Comment: 7 pages, 8 figure
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