24 research outputs found
Aging and Energy Landscapes: Application to Liquids and Glasses
The equation of state for a liquid in equilibrium, written in the potential
energy landscape formalism, is generalized to describe out-of-equilibrium
conditions. The hypothesis that during aging the system explores basins
associated to equilibrium configurations is the key ingredient in the
derivation. Theoretical predictions are successfully compared with data from
molecular dynamics simulations of different aging processes, such as
temperature and pressure jumps.Comment: RevTeX4, 4 pages, 5 eps figure
Metabasin dynamics and local structure in supercooled water
We employ the Distance Matrix method to investigate metabasin dynamics in
supercooled water. We find that the motion of the system consists in the
exploration of a finite region of configuration space (enclosing several
distinct local minima), named metabasin, followed by a sharp crossing to a
different metabasin. The characteristic time between metabasin transitions is
comparable to the structural relaxation time, suggesting that these transitions
are relevant for the long time dynamics. The crossing between metabasins is
accompanied by very rapid diffusional jumps of several groups of dynamically
correlated particles. These particles form relatively compact clusters and act
as cooperative relaxing units responsible for the density relaxation. We find
that these mobile particles are often characterized by an average coordination
larger than four, i.e. are located in regions where the tetrahedral hydrogen
bond network is distorted
Investigation of the relation between local diffusivity and local inherent structures in the Kob-Andersen Lennard-Jones model
We analyze one thousand independent equilibrium trajectories of a system of
155 Lennard Jones particles to separate in a model-free approach the role of
temperature and the role of the explored potential energy landscape basin depth
in the particle dynamics. We show that the diffusion coefficient can be
estimated as a sum over over contributions of the sampled basins, establishing
a connection between thermodynamics and dynamics in the potential energy
landscape framework. We provide evidence that the observed non-linearity in the
relation between local diffusion and basin depth is responsible for the
peculiar dynamic behavior observed in supercooled states and provide an
interpretation for the presence of dynamic heterogeneities.Comment: minor text changes, references adde
Configuration space connectivity across the fragile to strong transition in silica
We present a numerical analysis for SiO_2 of the fraction of diffusive
direction f_diff for temperatures T on both sides of the fragile-to-strong
crossover. The T-dependence of f_diff clearly reveals this change in dynamical
behavior. We find that for T above the crossover (fragile region) the system is
always close to ridges of the potential energy surface (PES), while below the
crossover (strong region), the system mostly explores the PES local minima.
Despite this difference, the power law dependence of f_diff on the diffusion
constant, as well as the power law dependence of f_diff on the configurational
entropy, shows no change at the fragile to strong crossover
Maximum Valency Lattice Gas Models
We study lattice gas models with the imposition of a constraint on the
maximum number of bonds (nearest neighbor interactions) that particles can
participate in. The critical parameters, as well as the coexistence region are
studied using the mean field approximation and the Bethe-Peierls approximation.
We find that the reduction of the number of interactions suppresses the
temperature-density region where the liquid and gas phases coexist. We confirm
these results from simulations using the histogram reweighting method employing
grand Canonical Monte Carlo simulations
Physics of the liquid-liquid critical point
Within the inherent structure (IS) thermodynamic formalism introduced by
Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys. Rev. A {\bf 25},
978 (1982)] we address the basic question of the physics of the liquid-liquid
transition and of density maxima observed in some complex liquids such as water
by identifying, for the first time, the statistical properties of the potential
energy landscape (PEL) responsible for these anomalies.
We also provide evidence of the connection between density anomalies and the
liquid-liquid critical point. Within the simple (and physically transparent)
model discussed, density anomalies do imply the existence of a liquid-liquid
transition.Comment: Physical Review Letters, in publicatio
Energy landscapes, ideal glasses, and their equation of state
Using the inherent structure formalism originally proposed by Stillinger and
Weber [Phys. Rev. A 25, 978 (1982)], we generalize the thermodynamics of an
energy landscape that has an ideal glass transition and derive the consequences
for its equation of state. In doing so, we identify a separation of
configurational and vibrational contributions to the pressure that corresponds
with simulation studies performed in the inherent structure formalism. We
develop an elementary model of landscapes appropriate to simple liquids which
is based on the scaling properties of the soft-sphere potential complemented
with a mean-field attraction. The resulting equation of state provides an
accurate representation of simulation data for the Lennard-Jones fluid,
suggesting the usefulness of a landscape-based formulation of supercooled
liquid thermodynamics. Finally, we consider the implications of both the
general theory and the model with respect to the so-called Sastry density and
the ideal glass transition. Our analysis shows that a quantitative connection
can be made between properties of the landscape and a simulation-determined
Sastry density, and it emphasizes the distinction between an ideal glass
transition and a Kauzmann equal-entropy condition.Comment: 11 pages, 3 figure
Configurational Entropy and Diffusivity of Supercooled Water
We calculate the configurational entropy S_conf for the SPC/E model of water
for state points covering a large region of the (T,rho) plane. We find that (i)
the (T,rho) dependence of S_conf correlates with the diffusion constant and
(ii) that the line of maxima in S_conf tracks the line of density maxima. Our
simulation data indicate that the dynamics are strongly influenced by S_conf
even above the mode-coupling temperature T_MCT(rho).Comment: Significant update of reference
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
Thermodynamic and structural aspects of the potential energy surface of simulated water
Relations between the thermodynamics and dynamics of supercooled liquids
approaching a glass transition have been proposed over many years. The
potential energy surface of model liquids has been increasingly studied since
it provides a connection between the configurational component of the partition
function on one hand, and the system dynamics on the other. This connection is
most obvious at low temperatures, where the motion of the system can be
partitioned into vibrations within a basin of attraction and infrequent
inter-basin transitions. In this work, we present a description of the
potential energy surface properties of supercooled liquid water. The dynamics
of this model has been studied in great details in the last years.
Specifically, we locate the minima sampled by the liquid by ``quenches'' from
equilibrium configurations generated via molecular dynamics simulations. We
calculate the temperature and density dependence of the basin energy,
degeneracy, and shape. The temperature dependence of the energy of the minima
is qualitatively similar to simple liquids, but has anomalous density
dependence. The unusual density dependence is also reflected in the
configurational entropy, the thermodynamic measure of degeneracy. Finally, we
study the structure of simulated water at the minima, which provides insight on
the progressive tetrahedral ordering of the liquid on cooling