206 research outputs found
Simulation-based equation of state of the hard disk fluid and prediction of higher-order virial coefficients
We present new molecular dynamics results for the pressure of the pure hard
disk fluid up to the hexatic transition (about reduced density 0.9). The data
combined with the known virial coefficients (up to ) are used to build
an equation of state, to estimate higher-order virial coefficients, and also to
obtain a better value of . Finite size effects are discussed in detail.
The ``van der Waals-like'' loop reported in literature in the vicinity of the
fluid/hexatic transition is explained by suppressed density fluctuations in the
canonical ensemble. The inflection point on the pressure-density dependence is
predicted by the equation of state even if the hexatic phase simulation data
are not considered.Comment: 9 pages, 3 figures, presented at The Seventh Liblice Conference on
the Statistical Mechanics of Liquids (Lednice, Czech Republic, June 11--16,
2006
Nonanalytical equation of state of the hard sphere fluid
An equation of state of the hard sphere fluid which is not analytical at the
freezing density is proposed and tested. The nonanalytical term is based on the
the classical nucleation theory and is able to capture the observed ``anomalous
increase'' of pressure at high densities. It is combined with the virial
expansion at low densities.Comment: 5 pages, 3 figure
Optimized Periodic Coulomb Potential in Two Dimension
The 1/r Coulomb potential is calculated for a two dimensional system with
periodic boundary conditions. Using polynomial splines in real space and a
summation in reciprocal space we obtain numerically optimized potentials which
allow us efficient calculations of any periodic (long-ranged) potential up to
high precision. We discuss the parameter space of the optimized potential for
the periodic Coulomb potential. Compared to the analytic Ewald potential, the
optimized potentials can reach higher precisions by up to several orders of
magnitude. We explicitly give simple expressions for fast calculations of the
periodic Coulomb potential where the summation in reciprocal space is reduced
to a few terms
Glassiness in Simple Liquids
In previous work the parameter of glassiness was introduced to distinguish
between a liquid and a glass, using a formal analogy with the quantum Bose
system. The glassiness is defined in such a way that it is unity in a frozen
system and less than one in a liquid. In the present letter we revise first the
results obtained for the glassiness in a hard sphere liquid as a function of
the density. Then we investigate the influence of an attractive potential by
obtaining the glassiness as a function of the density, temperature and the
attractive tail when a square well potential is added to the hard core.Comment: 8 pages, 4 figure
Polarization effects at the surface of aqueous alkali halide solutions
The polarizability of ions, with its strong influence on their surface affinity, is one of the crucial pieces of the complex puzzle that determines the surface properties of electrolyte solutions. Here, we investigate the electrical and structural properties of alkali halide solutions at a concentration of about 1.3 M using molecular dynamics simulations of polarizable water and ions models. We show that capillary fluctuations have a dramatic impact on the sampled quantities and that without removing their smearing effect, it would be impossible to resolve the local structure of the interfacial region. This procedure allows us to investigate in detail the dependence of the permanent and induced dipoles on the distance from the interface. The enhanced resolution gives us access to the surface charges, estimated using the Gouy-Chapman theory, despite the Debye length being shorter than the amplitude of capillary fluctuations
On extrapolation of virial coefficients of hard spheres
Several methods of extrapolating the virial coefficients, including those
proposed in this work, are discussed. The methods are demonstrated on
predicting higher virial coefficients of one-component hard spheres. Estimated
values of the eleventh to fifteenth virial coefficients are suggested. It has
been speculated that the virial coefficients, B_n, beyond B_{14} may decrease
with increasing n, and may reach negative values at large n. The extrapolation
techniques may be utilized in other fields of science where the art of
extrapolation plays a role.Comment: 8 pages, 1 figur
"The numerical accuracy of truncated Ewald sums for periodic systems with long-range Coulomb interactions"
Ewald summation is widely used to calculate electrostatic interactions in
computer simulations of condensed-matter systems. We present an analysis of the
errors arising from truncating the infinite real- and Fourier-space lattice
sums in the Ewald formulation. We derive an optimal choice for the
Fourier-space cutoff given a screening parameter . We find that the
number of vectors in Fourier space required to achieve a given accuracy scales
with . The proposed method can be used to determine computationally
efficient parameters for Ewald sums, to assess the quality of Ewald-sum
implementations, and to compare different implementations.Comment: 6 pages, 3 figures (Encapsulated PostScript), LaTe
Optimum Monte Carlo Simulations: Some Exact Results
We obtain exact results for the acceptance ratio and mean squared
displacement in Monte Carlo simulations of the simple harmonic oscillator in
dimensions. When the trial displacement is made uniformly in the radius, we
demonstrate that the results are independent of the dimensionality of the
space. We also study the dynamics of the process via a spectral analysis and we
obtain an accurate description for the relaxation time.Comment: 17 pages, 8 figures. submitted to J. Phys.
Chemical potential of quadrupolar two-centre Lennard-Jones fluids by gradual insertion
The gradual insertion method for direct calculation of the chemical potential
by molecular simulation is applied in the NpT ensemble to different quadrupolar
two-centre Lennard-Jones fluids at high density state points. The results agree
well with Widom's test particle insertion but show at very high densities
significantly smaller statistical uncertainties. The gradual insertion method,
which is coupled here with preferential sampling, extends the density range
where reliable information on the chemical potential can be obtained.
Application details are reported
Logarithmic corrections in the aging of the fully-frustrated Ising model
We study the dynamics of the critical two-dimensional fully-frustrated Ising
model by means of Monte Carlo simulations. The dynamical exponent is estimated
at equilibrium and is shown to be compatible with the value . In a
second step, the system is prepared in the paramagnetic phase and then quenched
at its critical temperature . Numerical evidences for the existence of
logarithmic corrections in the aging regime are presented. These corrections
may be related to the topological defects observed in other fully-frustrated
models. The autocorrelation exponent is estimated to be as for the
Ising chain quenched at .Comment: 12 pages, 9 figure
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