1,144 research outputs found
Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters
We present a detailed study of the energetics of water clusters (HO)
with , comparing diffusion Monte Carlo (DMC) and approximate density
functional theory (DFT) with well converged coupled-cluster benchmarks. We use
the many-body decomposition of the total energy to classify the errors of DMC
and DFT into 1-body, 2-body and beyond-2-body components. Using both
equilibrium cluster configurations and thermal ensembles of configurations, we
find DMC to be uniformly much more accurate than DFT, partly because some of
the approximate functionals give poor 1-body distortion energies. Even when
these are corrected, DFT remains considerably less accurate than DMC. When both
1- and 2-body errors of DFT are corrected, some functionals compete in accuracy
with DMC; however, other functionals remain worse, showing that they suffer
from significant beyond-2-body errors. Combining the evidence presented here
with the recently demonstrated high accuracy of DMC for ice structures, we
suggest how DMC can now be used to provide benchmarks for larger clusters and
for bulk liquid water.Comment: 34 pages, 6 figure
Armenius Oemler (1827-1897)
Armenius Oemler was an outstanding resident of Savannah whose innovative ideas in truck-farming and the formation of the Oemler Oyster Company, the south\u27s first oyster packaging plant, brought forth new industry to Chatham County. His headings were numerous consisting of physician, scientist, author, agriculturalist, entomologist, 03.ptain, and businessman. He was an informative source for scientists worldwide and a scholar in his own rite.https://digitalcommons.georgiasouthern.edu/sav-bios-lane/1168/thumbnail.jp
Benchmarking the performance of Density Functional Theory and Point Charge Force Fields in their Description of sI Methane Hydrate against Diffusion Monte Carlo
High quality reference data from diffusion Monte Carlo calculations are
presented for bulk sI methane hydrate, a complex crystal exhibiting both
hydrogen-bond and dispersion dominated interactions. The performance of some
commonly used exchange-correlation functionals and all-atom point charge force
fields is evaluated. Our results show that none of the exchange-correlation
functionals tested are sufficient to describe both the energetics and the
structure of methane hydrate accurately, whilst the point charge force fields
perform badly in their description of the cohesive energy but fair well for the
dissociation energetics. By comparing to ice Ih, we show that a good prediction
of the volume and cohesive energies for the hydrate relies primarily on an
accurate description of the hydrogen bonded water framework, but that to
correctly predict stability of the hydrate with respect to dissociation to ice
Ih and methane gas, accuracy in the water-methane interaction is also required.
Our results highlight the difficulty that density functional theory faces in
describing both the hydrogen bonded water framework and the dispersion bound
methane.Comment: 8 pages, 4 figures, 1 table. Minor typos corrected and clarification
added in Method
Quantum Monte Carlo study of the Ne atom and the Ne+ ion
We report all-electron and pseudopotential calculations of the
ground-stateenergies of the neutral Ne atom and the Ne+ ion using the
variational and diffusion quantum Monte Carlo (DMC) methods. We investigate
different levels of Slater-Jastrow trial wave function: (i) using Hartree-Fock
orbitals, (ii) using orbitals optimized within a Monte Carlo procedure in the
presence of a Jastrow factor, and (iii) including backflow correlations in the
wave function. Small reductions in the total energy are obtained by optimizing
the orbitals, while more significant reductions are obtained by incorporating
backflow correlations. We study the finite-time-step and fixed-node biases in
the DMC energy and show that there is a strong tendency for these errors to
cancel when the first ionization potential (IP) is calculated. DMC gives highly
accurate values for the IP of Ne at all the levels of trial wave function that
we have considered
Continuum variational and diffusion quantum Monte Carlo calculations
This topical review describes the methodology of continuum variational and
diffusion quantum Monte Carlo calculations. These stochastic methods are based
on many-body wave functions and are capable of achieving very high accuracy.
The algorithms are intrinsically parallel and well-suited to petascale
computers, and the computational cost scales as a polynomial of the number of
particles. A guide to the systems and topics which have been investigated using
these methods is given. The bulk of the article is devoted to an overview of
the basic quantum Monte Carlo methods, the forms and optimisation of wave
functions, performing calculations within periodic boundary conditions, using
pseudopotentials, excited-state calculations, sources of calculational
inaccuracy, and calculating energy differences and forces
Raman Spectroscopy Applied to the Noninvasive Detection of Monosodium Urate Crystal Deposits
An off-the-shelf Raman Spectrometer (RS) was used to noninvasively determine the presence of monosodium urate (MSU) crystals on the metatarsophalangeal joint (MTPJ) of a single gout sufferer. The spectrum sourced from the clinically diagnosed gout sufferer was compared to that sourced from an age-matched healthy subject scanned using the same protocol. Minimal signal processing was conducted on both spectra. Peaks characteristic of MSU crystals were evident on the spectrum sourced from the gout sufferer and not on the spectrum from the healthy control
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