173 research outputs found
Melting of Polydisperse Hard Disks
The melting of a polydisperse hard disk system is investigated by Monte Carlo
simulations in the semigrand canonical ensemble. This is done in the context of
possible continuous melting by a dislocation unbinding mechanism, as an
extension of the 2D hard disk melting problem. We find that while there is
pronounced fractionation in polydispersity, the apparent density-polydispersity
gap does not increase in width, contrary to 3D polydisperse hard spheres. The
point where the Young's modulus is low enough for the dislocation unbinding to
occur moves with the apparent melting point, but stays within the density gap,
just like for the monodisperse hard disk system. Additionally, we find that
throughout the accessible polydispersity range, the bound dislocation-pair
concentration is high enough to affect the dislocation unbinding melting as
predicted by Kosterlitz, Thouless, Halperin, Nelson and Young.Comment: 6 pages, 6 figure
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
Influence of polymer excluded volume on the phase behavior of colloid-polymer mixtures
We determine the depletion-induced phase-behavior of hard sphere colloids and
interacting polymers by large-scale Monte Carlo simulations using very accurate
coarse-graining techniques. A comparison with standard Asakura-Oosawa model
theories and simulations shows that including excluded volume interactions
between polymers leads to qualitative differences in the phase diagrams. These
effects become increasingly important for larger relative polymer size. Our
simulations results agree quantitatively with recent experiments.Comment: 5 pages, 4 figures submitted to Physical Review Letter
Accurate simulation estimates of phase behaviour in ternary mixtures with prescribed composition
This paper describes an isobaric semi-grand canonical ensemble Monte Carlo
scheme for the accurate study of phase behaviour in ternary fluid mixtures
under the experimentally relevant conditions of prescribed pressure,
temperature and overall composition. It is shown how to tune the relative
chemical potentials of the individual components to target some requisite
overall composition and how, in regions of phase coexistence, to extract
accurate estimates for the compositions and phase fractions of individual
coexisting phases. The method is illustrated by tracking a path through the
composition space of a model ternary Lennard-Jones mixture.Comment: 6 pages, 3 figure
Elastic constants and the effect of strain on monovacancy concentration in fcc hard-sphere crystals
We investigate the free energy and the concentration of monovacancies in strained face-centered-cubic (fee) hard-sphere crystals for several densities at and above melting. We use the conventional molecular dynamics method for simulations and employ a bias insertion method to extract properties of a monovacancy. We study two distinct constant-volume strains, considering a simple shear and an orthogonal expansion and contraction. Strains are examined across the linear elastic region and include also some nonlinear elastic deformations. Second-order elastic constants are reported as a function of density. The concentration of monovacancies decreases as density increases for both strained and unstrained crystals. The effect of strain is to cause the monovacancy concentration to increase by up to 72% for the expansion-contraction strain at the largest deformation studied. The effect of the shear strain is considerably less, and produces an increase in monovacancy concentration of at most 9% for the conditions studied here.open5
Generalized Ensemble and Tempering Simulations: A Unified View
From the underlying Master equations we derive one-dimensional stochastic
processes that describe generalized ensemble simulations as well as tempering
(simulated and parallel) simulations. The representations obtained are either
in the form of a one-dimensional Fokker-Planck equation or a hopping process on
a one-dimensional chain. In particular, we discuss the conditions under which
these representations are valid approximate Markovian descriptions of the
random walk in order parameter or control parameter space. They allow a unified
discussion of the stationary distribution on, as well as of the stationary flow
across each space. We demonstrate that optimizing the flow is equivalent to
minimizing the first passage time for crossing the space, and discuss the
consequences of our results for optimizing simulations. Finally, we point out
the limitations of these representations under conditions of broken ergodicity.Comment: 11 pages Latex, 2 eps figures, revised version, typos corrected, PRE
in pres
Microscopic origins of the anomalous melting behaviour of high-pressure sodium
Recent experiments have shown that sodium, a prototype simple metal at
ambient conditions, exhibits unexpected complexity under high pressure. One of
the most puzzling phenomena in the behaviour of dense sodium is the
pressure-induced drop in its melting temperature, which extends from 1000 K at
~30GPa to as low as room temperature at ~120GPa. Despite significant
theoretical effort to understand the anomalous melting its origins have
remained unclear. In this work, we reconstruct the sodium phase diagram using
an ab-initio-quality neural-network potential. We demonstrate that the
reentrant behaviour results from the screening of interionic interactions by
conduction electrons, which at high pressure induces a softening in the
short-range repulsion. It is expected that such an effect plays an important
role in governing the behaviour of a wide range of metals and alloys.Comment: 5 pages, 4 figures, 30 references, supplementary informatio
Study of the low-temperature behavior of a disordered antiferromagnet with random fields by the parallel-tempering method
The parallel-tempering method has been applied to numerically study the
thermodynamic behavior of a three-dimensional disordered antiferromagnetic
Ising model with random fields at spin concentrations corresponding to regions
of both weak and strong structural disorder. An analysis of the low-temperature
behavior of the model convincingly shows that in the case of a weakly
disordered samples there is realized an antiferromagnetic ordered state, while
in the region of strong structural disorder the effects of random magnetic
fields lead to the realization of a new phase state of the system with a
complex domain structure consisting of antiferromagnetic and ferromagnetic
domains separated by regions of a spin-glass phase and characterized by a
spinglass ground state.Comment: 12 RevTeX pages, 8 figure
Temperature and density extrapolations in canonical ensemble Monte Carlo simulations
We show how to use the multiple histogram method to combine canonical
ensemble Monte Carlo simulations made at different temperatures and densities.
The method can be applied to study systems of particles with arbitrary
interaction potential and to compute the thermodynamic properties over a range
of temperatures and densities. The calculation of the Helmholtz free energy
relative to some thermodynamic reference state enables us to study phase
coexistence properties. We test the method on the Lennard-Jones fluids for
which many results are available.Comment: 5 pages, 3 figure
Communication: Analytic continuation of the virial series through the critical point using parametric approximants
The mathematical structure imposed by the thermodynamic critical point motivates an approximant that synthesizes two theoretically sound equations of state: the parametric and the virial. The former is constructed to describe the critical region, incorporating all scaling laws; the latter is an expansion about zero density, developed from molecular considerations. The approximant is shown to yield an equation of state capable of accurately describing properties over a large portion of the thermodynamic parameter space, far greater than that covered by each treatment alone
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