2,515 research outputs found
An improved Monte Carlo method for direct calculation of the density of states
We present an efficient Monte Carlo algorithm for determining the density of
states which is based on the statistics of transition probabilities between
states. By measuring the infinite temperature transition probabilities--that
is, the probabilities associated with move proposal only--we are able to
extract excellent estimates of the density of states. When this estimator is
used in conjunction with a Wang-Landau sampling scheme [F. Wang and D. P.
Landau, Phys. Rev. Lett. 86, 2050 (2001)], we quickly achieve uniform sampling
of macrostates (e.g., energies) and systematically refine the calculated
density of states. This approach requires only potential energy evaluations,
continues to improve the statistical quality of its results as the simulation
time is extended, and is applicable to both lattice and continuum systems. We
test the algorithm on the Lennard-Jones liquid and demonstrate good statistical
convergence properties.Comment: 7 pages, 4 figures. to appear in Journal of Chemical Physic
Structural Order in Glassy Water
We investigate structural order in glassy water by performing classical
molecular dynamics simulations using the extended simple point charge (SPC/E)
model of water. We perform isochoric cooling simulations across the glass
transition temperature at different cooling rates and densities. We quantify
structural order by orientational and translational order metrics. Upon cooling
the liquid into the glassy state, both the orientational order parameter
and translational order parameter increase. At T=0 K, the glasses fall
on a line in the - plane or {\it order map}.
The position of this line depends only on density and coincides with the
location in the order map of the inherent structures (IS) sampled upon cooling.
We evaluate the energy of the IS, , and find that both order
parameters for the IS are proportional to . We also study the
structural order during the transformation of low-density amorphous ice (LDA)
to high-density amorphous ice (HDA) upon isothermal compression and are able to
identify distinct regions in the order map corresponding to these glasses.
Comparison of the order parameters for LDA and HDA with those obtained upon
isochoric cooling indicates major structural differences between glasses
obtained by cooling and glasses obtained by compression. These structural
differences are only weakly reflected in the pair correlation function. We also
characterize the evolution of structural order upon isobaric annealing, leading
at high pressure to very-high density amorphous ice (VHDA).Comment: submitte
Spinodal of supercooled polarizable water
We develop a series of molecular dynamics computer simulations of liquid
water, performed with a polarizable potential model, to calculate the spinodal
line and the curve of maximum density inside the metastable supercooled region.
After analysing the structural properties,the liquid spinodal line is followed
down to T=210 K. A monotonic decrease is found in the explored region. The
curve of maximum density bends on approaching the spinodal line. These results,
in agreement with similar studies on non polarizable models of water, are
consistent with the existence of a second critical point for water.Comment: 8 pages, 5 figures, 2 tables. To be published in Phys. Re
Softness dependence of the Anomalies for the Continuous Shouldered Well potential
By molecular dynamic simulations we study a system of particles interacting
through a continuous isotropic pairwise core-softened potential consisting of a
repulsive shoulder and an attractive well. The model displays a phase diagram
with three fluid phases, a gas-liquid critical point, a liquid-liquid critical
point, and anomalies in density, diffusion and structure. The hierarchy of the
anomalies is the same as for water. We study the effect on the anomalies of
varying the softness of the potential. We find that, making the soft-core
steeper, the regions of density and diffusion anomalies contract in the T -
{\rho} plane, while the region of structural anomaly is weakly affected.
Therefore, a liquid can have anomalous structural behavior without density or
diffusion anomalies. We show that, by considering as effective distances those
corresponding to the maxima of the first two peaks of the radial distribution
function g(r) in the high-density liquid, we can generalize to continuous
two-scales potentials a criterion for the occurrence of the anomalies of
density and diffusion, originally proposed for discontinuous potentials. We
observe that the knowledge of the structural behavior within the first two
coordination shells of the liquid is not enough to establish the occurrence of
the anomalies. By introducing the density derivative of the the cumulative
order integral of the excess entropy we show that the anomalous behavior is
regulated by the structural order at distances as large as the fourth
coordination shell. By comparing the results for different softness of the
potential, we conclude that the disappearing of the density and diffusion
anomalies for the steeper potentials is due to a more structured short-range
order. All these results increase our understanding on how, knowing the
interaction potential, we can evaluate the possible presence of anomalies for a
liquid
Liquid-Liquid Phase Transitions for Soft-Core Attractive Potentials
Using event driven molecular dynamics simulations, we study a three
dimensional one-component system of spherical particles interacting via a
discontinuous potential combining a repulsive square soft core and an
attractive square well. In the case of a narrow attractive well, it has been
shown that this potential has two metastable gas-liquid critical points. Here
we systematically investigate how the changes of the parameters of this
potential affect the phase diagram of the system. We find a broad range of
potential parameters for which the system has both a gas-liquid critical point
and a liquid-liquid critical point. For the liquid-gas critical point we find
that the derivatives of the critical temperature and pressure, with respect to
the parameters of the potential, have the same signs: they are positive for
increasing width of the attractive well and negative for increasing width and
repulsive energy of the soft core. This result resembles the behavior of the
liquid-gas critical point for standard liquids. In contrast, for the
liquid-liquid critical point the critical pressure decreases as the critical
temperature increases. As a consequence, the liquid-liquid critical point
exists at positive pressures only in a finite range of parameters. We present a
modified van der Waals equation which qualitatively reproduces the behavior of
both critical points within some range of parameters, and give us insight on
the mechanisms ruling the dependence of the two critical points on the
potential's parameters. The soft core potential studied here resembles model
potentials used for colloids, proteins, and potentials that have been related
to liquid metals, raising an interesting possibility that a liquid-liquid phase
transition may be present in some systems where it has not yet been observed.Comment: 29 pages, 15 figure
Mpemba effect and phase transitions in the adiabatic cooling of water before freezing
An accurate experimental investigation on the Mpemba effect (that is, the
freezing of initially hot water before cold one) is carried out, showing that
in the adiabatic cooling of water a relevant role is played by supercooling as
well as by phase transitions taking place at 6 +/- 1 oC, 3.5 +/- 0.5 oC and 1.3
+/- 0.6 oC, respectively. The last transition, occurring with a non negligible
probability of 0.21, has not been detected earlier. Supported by the
experimental results achieved, a thorough theoretical analysis of supercooling
and such phase transitions, which are interpreted in terms of different
ordering of clusters of molecules in water, is given.Comment: revtex, 4 pages, 2 figure
Supersaturated dispersions of rod-like viruses with added attraction
The kinetics of isotropic-nematic (I-N) and nematic-isotropic (N-I) phase
transitions in dispersions of rod-like {\it fd}-viruses are studied.
Concentration quenches were applied using pressure jumps in combination with
polarization microscopy, birefringence and turbidity measurements. The full
biphasic region could be accessed, resulting in the construction of a first
experimental analogue of the bifurcation diagram. The N-I spinodal points for
dispersions of rods with varying concentrations of depletion agents (dextran)
were obtained from orientation quenches, using cessation of shear flow in
combination with small angle light scattering. We found that the location of
the N-I spinodal point is independent of the attraction, which was confirmed by
theoretical calculations. Surprisingly, the experiments showed that also the
absolute induction time, the critical nucleus and the growth rate are
insensitive of the attraction, when the concentration is scaled to the distance
to the phase boundaries.Comment: 13 pages, 14 figures. accepted in Phsical Review
Non-exponential kinetic behavior of confined water
We present the results of molecular dynamics simulations of SPC/E water
confined in a realistic model of a silica pore. The single-particle dynamics
have been studied at ambient temperature for different hydration levels. The
confinement near the hydrophilic surface makes the dynamic behaviour of the
liquid strongly dependent on the hydration level. Upon decrease of the number
of water molecules in the pore we observe the onset of a slow dynamics due to
the ``cage effect''. The conventional picture of a stochastic single-particle
diffusion process thus looses its validity
Potential Energy Landscape Equation of State
Depth, number, and shape of the basins of the potential energy landscape are
the key ingredients of the inherent structure thermodynamic formalism
introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys.
Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only
on the volume dependence of these landscape properties is derived. Vibrational
and configurational contributions to pressure are sorted out in a transparent
way. Predictions are successfully compared with data from extensive molecular
dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure
- …