1,085 research outputs found
Comment on 'Molybdenum at High Pressure and Temperature: Melting from Another Solid Phase'
There has been a major controversy over the past seven years about the
high-pressure melting curves of transition metals. Static compression
(diamond-anvil cell: DAC) experiments up to the Mbar region give very low
melting slopes dT_m/dP, but shock-wave (SW) data reveal transitions indicating
much larger dT_m/dP values. Ab initio calculations support the correctness of
the shock data. In a very recent letter, Belonoshko et al. propose a simple and
elegant resolution of this conflict for molybdenum. Using ab initio
calculations based on density functional theory (DFT), they show that the
high-P/high-T phase diagram of Mo must be more complex than was hitherto
thought. Their calculations give convincing evidence that there is a transition
boundary between the normal bcc structure of Mo and a high-T phase, which they
suggest could be fcc. They propose that this transition was misinterpreted as
melting in DAC experiments. In confirmation, they note that their boundary also
explains a transition seen in the SW data. We regard Belonoshko et al.'s Letter
as extremely important, but we note that it raises some puzzling questions, and
we believe that their proposed phase diagram cannot be completely correct. We
have calculated the Helmholtz and Gibbs free energies of the bcc, fcc and hcp
phases of Mo, using essentially the same quasiharmonic methods as used by
Belonoshko et al.; we find that at high-P and T Mo in the hcp structure is more
stable than in bcc or fcc.Comment: 1 page, 1 figure. submitted to Phys. Rev. Let
Ab initio melting curve of molybdenum by the phase coexistence method
We report ab initio calculations of the melting curve of molybdenum for the
pressure range 0-400 GPa. The calculations employ density functional theory
(DFT) with the Perdew-Burke-Ernzerhof exchange-correlation functional in the
projector augmented wave (PAW) implementation. We present tests showing that
these techniques accurately reproduce experimental data on low-temperature
b.c.c. Mo, and that PAW agrees closely with results from the full-potential
linearized augmented plane-wave implementation. The work attempts to overcome
the uncertainties inherent in earlier DFT calculations of the melting curve of
Mo, by using the ``reference coexistence'' technique to determine the melting
curve. In this technique, an empirical reference model (here, the embedded-atom
model) is accurately fitted to DFT molecular dynamics data on the liquid and
the high-temperature solid, the melting curve of the reference model is
determined by simulations of coexisting solid and liquid, and the ab initio
melting curve is obtained by applying free-energy corrections. Our calculated
melting curve agrees well with experiment at ambient pressure and is consistent
with shock data at high pressure, but does not agree with the high pressure
melting curve deduced from static compression experiments. Calculated results
for the radial distribution function show that the short-range atomic order of
the liquid is very similar to that of the high-T solid, with a slight decrease
of coordination number on passing from solid to liquid. The electronic
densities of states in the two phases show only small differences. The results
do not support a recent theory according to which very low dTm/dP values are
expected for b.c.c. transition metals because of electron redistribution
between s-p and d states.Comment: 27 pages, 10 figures. to be published in Journal of Chemical Physic
The kinetics of homogeneous melting beyond the limit of superheating
Molecular dynamics simulation is used to study the time-scales involved in
the homogeneous melting of a superheated crystal. The interaction model used is
an embedded-atom model for Fe developed in previous work, and the melting
process is simulated in the microcanonical ensemble. We study
periodically repeated systems containing from 96 to 7776 atoms, and the initial
system is always the perfect crystal without free surfaces or other defects.
For each chosen total energy and number of atoms , we perform several
hundred statistically independent simulations, with each simulation lasting for
between 500 ps and 10 ns, in order to gather statistics for the waiting time
before melting occurs. We find that the probability distribution
of is roughly exponential, and that the mean value depends strongly on the excess of the initial steady temperature of the
crystal above the superheating limit identified by other researchers. The mean
also depends strongly on system size in a way that we have
quantified. For very small systems of atoms, we observe a persistent
alternation between the solid and liquid states, and we explain why this
happens. Our results allow us to draw conclusions about the reliability of the
recently proposed Z method for determining the melting properties of simulated
materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure
Melting properties of a simple tight-binding model of transition metals: I.The region of half-filled d-band
We present calculations of the free energy, and hence the melting properties,
of a simple tight-binding model for transition metals in the region of d-band
filling near the middle of a d-series, the parameters of the model being
designed to mimic molybdenum. The melting properties are calculated for
pressures ranging from ambient to several Mbar. The model is intended to be the
simplest possible tight-binding representation of the two basic parts of the
energy: first, the pairwise repulsion due to Fermi exclusion; and second, the
d-band bonding energy described in terms of an electronic density of states
that depends on structure. In addition to the number of d-electrons, the model
contains four parameters, which are adjusted to fit the pressure dependent
d-band width and the zero-temperature pressure-volume relation of Mo. We show
that the resulting model reproduces well the phonon dispersion relations of Mo
in the body-centred-cubic structure, as well as the radial distribution
function of the high-temperature solid and liquid given by earlier
first-principles simulations. Our free-energy calculations start from the free
energy of the liquid and solid phases of the purely repulsive pair-potential
model, without d-band bonding. The free energy of the full tight-binding model
is obtained from this by thermodynamic integration. The resulting melting
properties of the model are quite close to those given by earlier
first-principles work on Mo. An interpretation of these melting properties is
provided by showing how they are related to those of the purely repulsive
model.Comment: 34 pages, 12 figures. Accepted for publication in Journal of Chemical
Physic
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
Melting curve and Hugoniot of molybdenum up to 400 GPa by ab initio simulations
We report ab initio calculations of the melting curve and Hugoniot of
molybdenum for the pressure range 0-400 GPa, using density functional theory
(DFT) in the projector augmented wave (PAW) implementation. We use the
``reference coexistence'' technique to overcome uncertainties inherent in
earlier DFT calculations of the melting curve of Mo. Our calculated melting
curve agrees well with experiment at ambient pressure and is consistent with
shock data at high pressure, but does not agree with the high pressure melting
curve from static compression experiments. Our calculated P(V) and T(P)
Hugoniot relations agree well with shock measurements. We use calculations of
phonon dispersion relations as a function of pressure to eliminate some
possible interpretations of the solid-solid phase transition observed in shock
experiments on Mo.Comment: 8 pages, 6 figure
Energy benchmarks for water clusters and ice structures from an embedded many-body expansion
We show how an embedded many-body expansion (EMBE) can be used to calculate
accurate \emph{ab initio} energies of water clusters and ice structures using
wavefunction-based methods. We use the EMBE described recently by Bygrave
\emph{et al.} (J. Chem. Phys. \textbf{137}, 164102 (2012)), in which the terms
in the expansion are obtained from calculations on monomers, dimers, etc. acted
on by an approximate representation of the embedding field due to all other
molecules in the system, this field being a sum of Coulomb and
exchange-repulsion fields. Our strategy is to separate the total energy of the
system into Hartree-Fock and correlation parts, using the EMBE only for the
correlation energy, with the Hartree-Fock energy calculated using standard
molecular quantum chemistry for clusters and plane-wave methods for crystals.
Our tests on a range of different water clusters up to the 16-mer show that for
the second-order M\o{}ller-Plesset (MP2) method the EMBE truncated at 2-body
level reproduces to better than 0.1 m/monomer the correlation energy
from standard methods. The use of EMBE for computing coupled-cluster energies
of clusters is also discussed. For the ice structures Ih, II and VIII, we find
that MP2 energies near the complete basis-set limit reproduce very well the
experimental values of the absolute and relative binding energies, but that the
use of coupled-cluster methods for many-body correlation (non-additive
dispersion) is essential for a full description. Possible future applications
of the EMBE approach are suggested
Quantum atomic delocalization vs. structural disorder in amorphous silicon
Quantum effects on the atom delocalization in amorphous silicon have been
studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum
delocalization is appreciable vs. topological disorder, as seen from structural
observables such as the radial distribution function (RDF). At low
temperatures, the width of the first peak in the RDF increases by a factor of
1.5 due to quantum effects. The overall anharmonicity of the solid vibrations
at finite temperatures in amorphous silicon is clearly larger than in the
crystalline material. Low-energy vibrational modes are mainly located on
coordination defects in the amorphous material.Comment: 5 pages, 5 PS figures, REVTE
Perspective: How good is DFT for water?
Kohn-Sham density functional theory (DFT) has become established as an
indispensable tool for investigating aqueous systems of all kinds, including
those important in chemistry, surface science, biology and the earth sciences.
Nevertheless, many widely used approximations for the exchange-correlation (XC)
functional describe the properties of pure water systems with an accuracy that
is not fully satisfactory. The explicit inclusion of dispersion interactions
generally improves the description, but there remain large disagreements
between the predictions of different dispersion-inclusive methods. We present
here a review of DFT work on water clusters, ice structures and liquid water,
with the aim of elucidating how the strengths and weaknesses of different XC
approximations manifest themselves across this variety of water systems. Our
review highlights the crucial role of dispersion in describing the delicate
balance between compact and extended structures of many different water
systems, including the liquid. By referring to a wide range of published work,
we argue that the correct description of exchange-overlap interactions is also
extremely important, so that the choice of semi-local or hybrid functional
employed in dispersion-inclusive methods is crucial. The origins and
consequences of beyond-2-body errors of approximate XC functionals are noted,
and we also discuss the substantial differences between different
representations of dispersion. We propose a simple numerical scoring system
that rates the performance of different XC functionals in describing water
systems, and we suggest possible future developments
Solid helium at high pressure: A path-integral Monte Carlo simulation
Solid helium (3He and 4He) in the hcp and fcc phases has been studied by
path-integral Monte Carlo. Simulations were carried out in the
isothermal-isobaric (NPT) ensemble at pressures up to 52 GPa. This allows one
to study the temperature and pressure dependences of isotopic effects on the
crystal volume and vibrational energy in a wide parameter range. The obtained
equation of state at room temperature agrees with available experimental data.
The kinetic energy, E_k, of solid helium is found to be larger than the
vibrational potential energy, E_p. The ratio E_k/E_p amounts to about 1.4 at
low pressures, and decreases as the applied pressure is raised, converging to
1, as in a harmonic solid. Results of these simulations have been compared with
those yielded by previous path integral simulations in the NVT ensemble. The
validity range of earlier approximations is discussed.Comment: 7 pages, 5 figure
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