2,427 research outputs found
A Thermodynamically-Consistent Non-Ideal Stochastic Hard-Sphere Fluid
A grid-free variant of the Direct Simulation Monte Carlo (DSMC) method is
proposed, named the Isotropic DSMC (I-DSMC) method, that is suitable for
simulating dense fluid flows at molecular scales. The I-DSMC algorithm
eliminates all grid artifacts from the traditional DSMC algorithm; it is
Galilean invariant and microscopically isotropic. The stochastic collision
rules in I-DSMC are modified to yield a non-ideal structure factor that gives
consistent compressibility, as first proposed in [Phys. Rev. Lett. 101:075902
(2008)]. The resulting Stochastic Hard Sphere Dynamics (SHSD) fluid is
empirically shown to be thermodynamically identical to a deterministic
Hamiltonian system of penetrable spheres interacting with a linear core pair
potential, well-described by the hypernetted chain (HNC) approximation. We
apply a stochastic Enskog kinetic theory for the SHSD fluid to obtain estimates
for the transport coefficients that are in excellent agreement with particle
simulations over a wide range of densities and collision rates. The fluctuating
hydrodynamic behavior of the SHSD fluid is verified by comparing its dynamic
structure factor against theory based on the Landau-Lifshitz Navier-Stokes
equations. We also study the Brownian motion of a nano-particle suspended in an
SHSD fluid and find a long-time power-law tail in its velocity autocorrelation
function consistent with hydrodynamic theory and molecular dynamics
calculations.Comment: 30 pages, revision adding some clarifications and a new figure. See
also arXiv:0803.035
Phase transition in inelastic disks
This letter investigates the molecular dynamics of inelastic disks without
external forcing. By introducing a new observation frame with a rescaled time,
we observe the virtual steady states converted from asymptotic energy
dissipation processes. System behavior in the thermodynamic limit is carefully
investigated. It is found that a phase transition with symmetry breaking occurs
when the magnitude of dissipation is greater than a critical value.Comment: 9 pages, 6 figure
Stacking Entropy of Hard Sphere Crystals
Classical hard spheres crystallize at equilibrium at high enough density.
Crystals made up of stackings of 2-dimensional hexagonal close-packed layers
(e.g. fcc, hcp, etc.) differ in entropy by only about per sphere
(all configurations are degenerate in energy). To readily resolve and study
these small entropy differences, we have implemented two different
multicanonical Monte Carlo algorithms that allow direct equilibration between
crystals with different stacking sequences. Recent work had demonstrated that
the fcc stacking has higher entropy than the hcp stacking. We have studied
other stackings to demonstrate that the fcc stacking does indeed have the
highest entropy of ALL possible stackings. The entropic interactions we could
detect involve three, four and (although with less statistical certainty) five
consecutive layers of spheres. These interlayer entropic interactions fall off
in strength with increasing distance, as expected; this fall-off appears to be
much slower near the melting density than at the maximum (close-packing)
density. At maximum density the entropy difference between fcc and hcp
stackings is per sphere, which is roughly 30% higher
than the same quantity measured near the melting transition.Comment: 15 page
Free energies of crystalline solids: a lattice-switch Monte Carlo method
We present a method for the direct evaluation of the difference between the
free energies of two crystalline structures, of different symmetry. The method
rests on a Monte Carlo procedure which allows one to sample along a path,
through atomic-displacement-space, leading from one structure to the other by
way of an intervening transformation that switches one set of lattice vectors
for another. The configurations of both structures can thus be sampled within a
single Monte Carlo process, and the difference between their free energies
evaluated directly from the ratio of the measured probabilities of each. The
method is used to determine the difference between the free energies of the fcc
and hcp crystalline phases of a system of hard spheres.Comment: 5 pages Revtex, 3 figure
Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System
The long-time behavior of the velocity autocorrelation function (VACF) is
investigated by the molecular dynamics simulation of a two-dimensional system
which has both a many-body interaction and a random potential. With
strengthening the random potential by increasing the density of impurities, a
crossover behavior of the VACF is observed from a positive tail, which is
proportional to t^{-1}, to a negative tail, proportional to -t^{-2}. The latter
tail exists even when the density of particles is the same order as the density
of impurities. The behavior of the VACF in a nonequilibrium steady state is
also studied. In the linear response regime the behavior is similar to that in
the equilibrium state, whereas it changes drastically in the nonlinear response
regime.Comment: 12 pages, 5 figure
Food Preservation--Plan Ahead
Publication gives instructions on how to prepare for preserving your own food, including equipment needed, preparation instructions, and hints for a successful preservation process
Coulomb Excitation of Multi-Phonon Levels of the Giant Dipole Resonance
A closed expression is obtained for the cross-section for Coulomb excitation
of levels of the giant dipole resonance of given angular momentum and phonon
number. Applications are made to the Goldhaber-Teller and Steinwedel-Jensen
descriptions of the resonance, at non-relativistic and relativistic bombarding
energies.Comment: 16 pages, 5 figure
A Consistent Boltzmann Algorithm
The direct simulation Monte Carlo method for the Boltzmann equation is modified by an additional displacement in the advection process and an enhanced collision rate in order to obtain the exact hard sphere equation of state at all densities. This leads to consistent thermodynamic and transport properties in the low density (Boltzmann) regime. At higher densities transport properties are comparable to the predictions of the Enskog model. The algorithm is faster than molecular dynamics at low and moderate densities and readily run on a parallel architectur
- …