468 research outputs found
Evaluation of configurational entropy of a model liquid from computer simulations
Computer simulations have been employed in recent years to evaluate the
configurational entropy changes in model glass-forming liquids. We consider two
methods, both of which involve the calculation of the `intra-basin' entropy as
a means for obtaining the configurational entropy. The first method involves
the evaluation of the intra-basin entropy from the vibrational frequencies of
inherent structures, by making a harmonic approximation of the local potential
energy topography. The second method employs simulations that confine the
liquid within a localized region of configuration space by the imposition of
constraints; apart from the choice of the constraints, no further assumptions
are made. We compare the configurational entropies estimated for a model liquid
(binary mixture of particles interacting {\it via} the Lennard-Jones potential)
for a range of temperatures, at fixed density.Comment: 10 pages, 5 figures, Proceedings of "Unifying Concepts in Glass
Physics" Trieste 1999 (to appear in J. Phys. Cond. Mat.
Landscapes, dynamic heterogeneity and kinetic facilitation in a simple off-lattice model
We present a simple off-lattice hard-disc model that exhibits glassy
dynamics. The inherent structures are enumerated exactly, transitions between
metabasins are well understood, and the particle configurations that act to
facilitate dynamics are easily identified. The model readily maps to a coarse
grained dynamic facilitation description.Comment: 5 pages, 5 figures, submitted to PR
On the liquid-glass transition line in monatomic Lennard-Jones fluids
A thermodynamic approach to derive the liquid-glass transition line in the
reduced temperature vs reduced density plane for a monatomic Lennard-Jones
fluid is presented. The approach makes use of a recent reformulation of the
classical perturbation theory of liquids [M. Robles and M. L\'opez de Haro,
Phys. Chem. Chem. Phys. {\bf 3}, 5528 (2001)] which is at grips with a rational
function approximation for the Laplace transform of the radial distribution
function of the hard-sphere fluid. The only input required is an equation of
state for the hard-sphere system. Within the Mansoori-Canfield/Rasaiah-Stell
variational perturbation theory, two choices for such an equation of state,
leading to a glass transition for the hard-sphere fluid, are considered. Good
agreement with the liquid-glass transition line derived from recent molecular
dynamic simulations [Di Leonardo et al., Phys. Rev. Lett. {\bf 84}, 6054(2000)]
is obtained.Comment: 4 pages, 2 figure
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
Diffusion in simple fluids
Computed self diffusion coefficients for the Lennard-Jones and hard sphere fluids are related by
Dej = DNs(aB) exp (--e/2kB T)
where σB=σLJ(2/[1+ii(1+2kBT/ε)])1/6, the effective hard sphere diameter, is the (average) distance of closest approach in collisions between molecules which interact with the positive part of the LJ potential, and the Arrhenius term reflects the influence of the negative part. σLJ and ε are the size and well depth parameters. Measured diffusion coefficients of the halomethane liquids are reproduced by the equation over wide ranges of temperature and density and do not reveal any influence of the inelastic effects associated with molecular anisotropy
Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids
The liquid-gas spinodal and the glass transition define ultimate boundaries
beyond which substances cannot exist as (stable or metastable) liquids. The
relation between these limits is analyzed {\it via} computer simulations of a
model liquid. The results obtained indicate that the liquid - gas spinodal and
the glass transition lines intersect at a finite temperature, implying a glass
- gas mechanical instability locus at low temperatures. The glass transition
lines obtained by thermodynamic and dynamic criteria agree very well with each
other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Two-dimensional lattice-fluid model with water-like anomalies
We investigate a lattice-fluid model defined on a two-dimensional triangular
lattice, with the aim of reproducing qualitatively some anomalous properties of
water. Model molecules are of the "Mercedes Benz" type, i.e., they possess a D3
(equilateral triangle) symmetry, with three bonding arms. Bond formation
depends both on orientation and local density. We work out phase diagrams,
response functions, and stability limits for the liquid phase, making use of a
generalized first order approximation on a triangle cluster, whose accuracy is
verified, in some cases, by Monte Carlo simulations. The phase diagram displays
one ordered (solid) phase which is less dense than the liquid one. At fixed
pressure the liquid phase response functions show the typical anomalous
behavior observed in liquid water, while, in the supercooled region, a
reentrant spinodal is observed.Comment: 9 pages, 1 table, 7 figure
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
- …