494 research outputs found
Density minimum and liquid-liquid phase transition
We present a high-resolution computer simulation study of the equation of
state of ST2 water, evaluating the liquid-state properties at 2718 state
points, and precisely locating the liquid-liquid critical point (LLCP)
occurring in this model. We are thereby able to reveal the interconnected set
of density anomalies, spinodal instabilities and response function extrema that
occur in the vicinity of a LLCP for the case of a realistic, off-lattice model
of a liquid with local tetrahedral order. In particular, we unambiguously
identify a density minimum in the liquid state, define its relationship to
other anomalies, and show that it arises due to the approach of the liquid
structure to a defect-free random tetrahedral network of hydrogen bonds.Comment: 5 pages, 4 figure
"Swarm relaxation": Equilibrating a large ensemble of computer simulations
It is common practice in molecular dynamics and Monte Carlo computer
simulations to run multiple, separately-initialized simulations in order to
improve the sampling of independent microstates. Here we examine the utility of
an extreme case of this strategy, in which we run a large ensemble of
independent simulations (a "swarm"), each of which is relaxed to equilibrium.
We show that if is of order , we can monitor the swarm's relaxation
to equilibrium, and confirm its attainment, within , where
is the equilibrium relaxation time. As soon as a swarm of this size
attains equilibrium, the ensemble of final microstates from each run is
sufficient for the evaluation of most equilibrium properties without further
sampling. This approach dramatically reduces the wall-clock time required,
compared to a single long simulation, by a factor of several hundred, at the
cost of an increase in the total computational effort by a small factor. It is
also well-suited to modern computing systems having thousands of processors,
and is a viable strategy for simulation studies that need to produce
high-precision results in a minimum of wall-clock time. We present results
obtained by applying this approach to several test cases.Comment: 12 pages. To appear in Eur. Phy. J. E, 201
Energy landscape of a simple model for strong liquids
We calculate the statistical properties of the energy landscape of a minimal
model for strong network-forming liquids. Dynamics and thermodynamic properties
of this model can be computed with arbitrary precision even at low
temperatures. A degenerate disordered ground state and logarithmic statistics
for the energy distribution are the landscape signatures of strong liquid
behavior. Differences from fragile liquid properties are attributed to the
presence of a discrete energy scale, provided by the particle bonds, and to the
intrinsic degeneracy of topologically disordered networks.Comment: Revised versio
Non-Gaussian energy landscape of a simple model for strong network-forming liquids: accurate evaluation of the configurational entropy
We present a numerical study of the statistical properties of the potential
energy landscape of a simple model for strong network-forming liquids. The
model is a system of spherical particles interacting through a square well
potential, with an additional constraint that limits the maximum number of
bonds, , per particle. Extensive simulations have been carried out
as a function of temperature, packing fraction, and . The dynamics
of this model are characterized by Arrhenius temperature dependence of the
transport coefficients and by nearly exponential relaxation of dynamic
correlators, i.e. features defining strong glass-forming liquids. This model
has two important features: (i) landscape basins can be associated with bonding
patterns; (ii) the configurational volume of the basin can be evaluated in a
formally exact way, and numerically with arbitrary precision. These features
allow us to evaluate the number of different topologies the bonding pattern can
adopt. We find that the number of fully bonded configurations, i.e.
configurations in which all particles are bonded to neighbors, is
extensive, suggesting that the configurational entropy of the low temperature
fluid is finite. We also evaluate the energy dependence of the configurational
entropy close to the fully bonded state, and show that it follows a logarithmic
functional form, differently from the quadratic dependence characterizing
fragile liquids. We suggest that the presence of a discrete energy scale,
provided by the particle bonds, and the intrinsic degeneracy of fully bonded
disordered networks differentiates strong from fragile behavior.Comment: Final version. Journal of Chemical Physics 124, 204509 (2006
Stochastic Model and Equivalent Ferromagnetic Spin Chain with Alternation
We investigate a non-equilibrium reaction-diffusion model and equivalent
ferromagnetic spin 1/2 XY spin chain with alternating coupling constant. The
exact energy spectrum and the n-point hole correlations are considered with the
help of the Jordan-Wigner fermionization and the inter-particle distribution
function method. Although the Hamiltonian has no explicit translational
symmetry, the translational invariance is recovered after long time due to the
diffusion. We see the scaling relations for the concentration and the two-point
function in finite size analysis.Comment: 7 pages, LaTeX file, to appear in J. Phys. A: Math. and Ge
Distributions of inherent structure energies during aging
We perform extensive simulations of a binary mixture Lennard-Jones system
subjected to a temperature jump in order to study the time evolution of
fluctuations during aging. Analyzing data from 1500 different aging
realizations, we calculate distributions of inherent structure energies for
different aging times and contrast them with equilibrium. We find that the
distributions initially become narrower and then widen as the system
equilibrates. For deep quenches, fluctuations in the glassy system differ
significantly from those observed in equilibrium. Simulation results are
partially captured by theoretical predictions only when the final temperature
is higher than the mode coupling temperature.Comment: 5 pages, 4 figure
Mode-coupling theory predictions for a limited valency attractive square-well model
Recently we have studied, using numerical simulations, a limited valency
model, i.e. an attractive square well model with a constraint on the maximum
number of bonded neighbors. Studying a large region of temperatures and
packing fractions , we have estimated the location of the liquid-gas
phase separation spinodal and the loci of dynamic arrest, where the system is
trapped in a disordered non-ergodic state. Two distinct arrest lines for the
system are present in the system: a {\it (repulsive) glass} line at high
packing fraction, and a {\it gel} line at low and . The former is
essentially vertical (-controlled), while the latter is rather horizontal
(-controlled) in the plane. We here complement the molecular
dynamics results with mode coupling theory calculations, using the numerical
structure factors as input. We find that the theory predicts a repulsive glass
line -- in satisfactory agreement with the simulation results -- and an
attractive glass line which appears to be unrelated to the gel line.Comment: 12 pages, 6 figures. To appear in J. Phys. Condens. Matter, special
issue: "Topics in Application of Scattering Methods for Investigation of
Structure and Dynamics of Soft Condensed Matter", Fiesole, November 200
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