1,312 research outputs found
Ab initio statistical mechanics of surface adsorption and desorption: I. HO on MgO (001) at low coverage
We present a general computational scheme based on molecular dynamics (m.d.)
simulation for calculating the chemical potential of adsorbed molecules in
thermal equilibrium on the surface of a material. The scheme is based on the
calculation of the mean force in m.d. simulations in which the height of a
chosen molecule above the surface is constrained, and subsequent integration of
the mean force to obtain the potential of mean force and hence the chemical
potential. The scheme is valid at any coverage and temperature, so that in
principle it allows the calculation of the chemical potential as a function of
coverage and temperature. It avoids all statistical mechanical approximations,
except for the use of classical statistical mechanics for the nuclei, and
assumes nothing in advance about the adsorption sites. From the chemical
potential, the absolute desorption rate of the molecules can be computed,
provided the equilibration rate on the surface is faster than the desorption
rate. We apply the theory by {\em ab initio} m.d. simulation to the case of
HO on MgO (001) in the low-coverage limit, using the Perdew-Burke-Ernzerhof
(PBE) form of exchange-correlation. The calculations yield an {\em ab initio}
value of the Polanyi-Wigner frequency prefactor, which is more than two orders
of magnitude greater than the value of s often assumed in the
past. Provisional comparison with experiment suggests that the PBE adsorption
energy may be too low, but the extension of the calculations to higher
coverages is needed before firm conclusions can be drawn. The possibility of
including quantum nuclear effects by using path-integral simulations is noted.Comment: 11 pages + 10 figure
British naval policy in the 1920s
Concludes that government naval policy decisions in the 1920s had a direct bearing on the selection of appeasement as a diplomatic tool by British decision makers in the 1930s.Thesis (M.A.) -- University of Adelaide, Dept. of History, 200
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
High order Chin actions in path integral Monte Carlo
High order actions proposed by Chin have been used for the first time in path
integral Monte Carlo simulations. Contrarily to the Takahashi-Imada action,
which is accurate to fourth order only for the trace, the Chin action is fully
fourth order, with the additional advantage that the leading fourth and sixth
order error coefficients are finely tunable. By optimizing two free parameters
entering in the new action we show that the time step error dependence achieved
is best fitted with a sixth order law. The computational effort per bead is
increased but the total number of beads is greatly reduced, and the efficiency
improvement with respect to the primitive approximation is approximately a
factor of ten. The Chin action is tested in a one-dimensional harmonic
oscillator, a H drop, and bulk liquid He. In all cases a sixth-order
law is obtained with values of the number of beads that compare well with the
pair action approximation in the stringent test of superfluid He.Comment: 19 pages, 8 figure
The space station: Human factors and productivity
Human factor researchers and engineers are making inputs into the early stages of the design of the Space Station to improve both the quality of life and work on-orbit. Effective integration of the human factors information related to various Intravehicular Activity (IVA), Extravehicular Activity (EVA), and teletobotics systems during the Space Station design will result in increased productivity, increased flexibility of the Space Stations systems, lower cost of operations, improved reliability, and increased safety for the crew onboard the Space Station. The major features of productivity examined include the cognitive and physical effort involved in work, the accuracy of worker output and ability to maintain performance at a high level of accuracy, the speed and temporal efficiency with which a worker performs, crewmember satisfaction with their work environment, and the relation between performance and cost
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