6,763 research outputs found
Efficient configurational-bias Monte-Carlo simulations of chain molecules with `swarms' of trial configurations
Proposed here is a dynamic Monte-Carlo algorithm that is efficient in
simulating dense systems of long flexible chain molecules. It expands on the
configurational-bias Monte-Carlo method through the simultaneous generation of
a large set of trial configurations. This process is directed by attempting to
terminate unfinished chains with a low statistical weight, and replacing these
chains with clones (enrichments) of stronger chains. The efficiency of the
resulting method is explored by simulating dense polymer brushes. A gain in
efficiency of at least three orders of magnitude is observed with respect to
the configurational-bias approach, and almost one order of magnitude with
respect to recoil-growth Monte-Carlo. Furthermore, the inclusion of `waste
recycling' is observed to be a powerful method for extracting meaningful
statistics from the discarded configurations
Sampling along reaction coordinates with the Wang-Landau method
The multiple range random walk algorithm recently proposed by Wang and Landau
[Phys. Rev. Lett. 86, 2050 (2001)] is adapted to the computation of free energy
profiles for molecular systems along reaction coordinates. More generally, we
show how to extract partial averages in various statistical ensembles without
invoking simulations with constraints, biasing potentials or unknown
parameters. The method is illustrated on a model 10-dimensional potential
energy surface, for which analytical results are obtained. It is then applied
to the potential of mean force associated with the dihedral angle of the butane
molecule in gas phase and in carbon tetrachloride solvent. Finally,
isomerization in a small rocksalt cluster, Na4F4, is investigated in the
microcanonical ensemble, and the results are compared to those of parallel
tempering Monte Carlo.Comment: 6 pages, 5 figure
The role of long-range forces in the phase behavior of colloids and proteins
The phase behavior of colloid-polymer mixtures, and of solutions of globular
proteins, is often interpreted in terms of a simple model of hard spheres with
short-ranged attraction. While such a model yields a qualitative understanding
of the generic phase diagrams of both colloids and proteins, it fails to
capture one important difference: the model predicts fluid-fluid phase
separation in the metastable regime below the freezing curve. Such demixing has
been observed for globular proteins, but for colloids it appears to be
pre-empted by the appearance of a gel. In this paper, we study the effect of
additional long-range attractions on the phase behavior of spheres with
short-ranged attraction. We find that such attractions can shift the
(metastable) fluid-fluid critical point out of the gel region. As this
metastable critical point may be important for crystal nucleation, our results
suggest that long-ranged attractive forces may play an important role in the
crystallization of globular proteins. However, in colloids, where refractive
index matching is often used to switch off long-ranged dispersion forces,
gelation is likely to inhibit phase separation.Comment: EURO-LATEX, 6 pages, 2 figure
Equation of state of metallic hydrogen from Coupled Electron-Ion Monte Carlo simulations
We present a study of hydrogen at pressures higher than molecular
dissociation using the Coupled Electron-Ion Monte Carlo method. These
calculations use the accurate Reptation Quantum Monte Carlo method to estimate
the electronic energy and pressure while doing a Monte Carlo simulation of the
protons. In addition to presenting simulation results for the equation of state
over a large region of phase space, we report the free energy obtained by
thermodynamic integration. We find very good agreement with DFT calculations
for pressures beyond 600 GPa and densities above . Both
thermodynamic as well as structural properties are accurately reproduced by DFT
calculations. This agreement gives a strong support to the different
approximations employed in DFT, specifically the approximate
exchange-correlation potential and the use of pseudopotentials for the range of
densities considered. We find disagreement with chemical models, which suggests
a reinvestigation of planetary models, previously constructed using the
Saumon-Chabrier-Van Horn equations of state.Comment: 9 pages, 7 figure
Vapor-liquid surface tension of strong short-range Yukawa fluid
The thermodynamic properties of strong short-range attractive Yukawa fluids,
k=10, 9, 8, and 7, are determined by combining the slab technique with the
standard and the replica exchange Monte Carlo (REMC) methods. A good agreement
was found among the coexistence curves of these systems calculated by REMC and
those previously reported in the literature. However, REMC allows exploring the
coexistence at lower temperatures, where dynamics turns glassy. To obtain the
surface tension we employed, for both methods, a procedure that yields the
pressure tensor components for discontinuous potentials. The surface tension
results obtained by the standard MC and REMC techniques are in good agreement.Comment: 6 pages, 4 figure
Anomalous Dynamic Arrest in a Mixture of Big and Small Particles
We present molecular dynamics simulations on the slow dynamics of a mixture
of big and small soft-spheres with a large size disparity. Dynamics are
investigated in a broad range of temperature and mixture composition. As a
consequence of large size disparity, big and small particles exhibit very
different relaxation times. As previously reported for simple models of
short-ranged attractive colloids and polymer blends, several anomalous dynamic
features are observed: i) sublinear behavior for mean squared displacements,
ii) concave-to-convex crossover for density-density correlators, by varying
temperature or wavevector, iii) logarithmic decay for specific wavevectors of
density-density correlators. These anomalous features are observed over time
intervals extending up to four decades, and strongly resemble predictions of
the Mode Coupling Theory (MCT) for state points close to higher-order MCT
transitions, which originate from the competition between different mechanisms
for dynamic arrest. For the big particles we suggest competition between
soft-sphere repulsion and depletion effects induced by neighboring small
particles. For the small particles we suggest competition between bulk-like
dynamics and confinement, respectively induced by neighboring small particles
and by the slow matrix of big particles. By increasing the size disparity, a
new relaxation scenario arises for the small particles. Self-correlators decay
to zero at temperatures where density-density correlations are frozen. The
behavior of the latters resembles features characteristic of type-A MCT
transitions, defined by a zero value of the critical non-ergodicity parameter.Comment: Version 2. Added major new result
[N]pT ensemble and finite-size scaling study of the GEM-4 critical isostructural transition
First-order transitions of system where both lattice site occupancy and
lattice spacing fluctuate, such as cluster crystals, cannot be efficiently
studied by traditional simulation methods. These methods necessarily fix one of
these two degrees of freedom, but this difficulty is surmounted by the
generalized [N]pT ensemble [J. Chem. Phys. 136, 214106 (2012)]. Here it is
shown that histogram reweighting and the [N]pT ensemble can be used to study an
isostructural transition between cluster crystals of different occupancy in the
generalized exponential model of index 4 (GEM-4). Extending this scheme to
finite-size scaling studies also allows to accurately determine the critical
point parameters and to verify that it belongs to the Ising universality class.Comment: 5 pages, 4 figure
Transport properties of room temperature ionic liquids from classical molecular dynamics
Room Temperature Ionic Liquids (RTILs) have attracted much of the attention
of the scientific community in the past decade due the their novel and highly
customizable properties. Nonetheless their high viscosities pose serious
limitations to the use of RTILs in practical applications. To elucidate some of
the physical aspects behind transport properties of RTILs, extensive classical
molecular dynamics (MD) calculations are reported. Bulk viscosities and ionic
conductivities of butyl-methyl-imidazole based RTILs are presented over a wide
range of temperatures. The dependence of the properties of the liquids on
simulation parameters, e.g. system size effects and choice of the interaction
potential, is analyzed
Janssen effect and the stability of quasi 2-D sandpiles
We present the results of three dimensional molecular dynamics study of
global normal stresses in quasi two dimensional sandpiles formed by pouring
mono dispersed cohesionless spherical grains into a vertical granular Hele-Shaw
cell. We observe Janssen effect which is the phenomenon of pressure saturation
at the bottom of the container. Simulation of cells with different thicknesses
shows that the Janssen coefficient is a function of the cell
thickness. Dependence of global normal stresses as well as on the
friction coefficients between the grains () and with walls () are
also studied. The results show that in the range of our simulations
usually increases with wall-grain friction coefficient. Meanwhile by increasing
while the other system parameters are fixed, we witness a gradual
increase in to a parameter dependent maximal value
Using bijective maps to improve free energy estimates
We derive a fluctuation theorem for generalized work distributions, related
to bijective mappings of the phase spaces of two physical systems, and use it
to derive a two-sided constraint maximum likelihood estimator of their free
energy difference which uses samples from the equilibrium configurations of
both systems. As an application, we evaluate the chemical potential of a dense
Lennard-Jones fluid and study the construction and performance of suitable
maps.Comment: 17 pages, 11 figure
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