1,456 research outputs found
Slow dynamics in a primitive tetrahedral network model
We report extensive Monte Carlo and event-driven molecular dynamics
simulations of the fluid and liquid phase of a primitive model for silica
recently introduced by Ford, Auerbach and Monson [J. Chem. Phys. 17, 8415
(2004)]. We evaluate the iso-diffusivity lines in the temperature-density plane
to provide an indication of the shape of the glass transition line. Except for
large densities, arrest is driven by the onset of the tetrahedral bonding
pattern and the resulting dynamics is strong in the Angell's classification
scheme. We compare structural and dynamic properties with corresponding results
of two recently studied primitive models of network forming liquids -- a
primitive model for water and a angular-constraint free model of
four-coordinated particles -- to pin down the role of the geometric constraints
associated to the bonding. Eventually we discuss the similarities between
"glass" formation in network forming liquids and "gel" formation in colloidal
dispersions of patchy particles.Comment: 9 pages, 10 figure
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
Noro-Frenkel scaling in short-range square well: A Potential Energy Landscape study
We study the statistical properties of the potential energy landscape of a
system of particles interacting via a very short-range square-well potential
(of depth ), as a function of the range of attraction to provide
thermodynamic insights of the Noro and Frenkel [ M.G. Noro and D. Frenkel,
J.Chem.Phys. {\bf 113}, 2941 (2000)] scaling. We exactly evaluate the basin
free energy and show that it can be separated into a {\it vibrational}
(-dependent) and a {\it floppy} (-independent) component. We
also show that the partition function is a function of ,
explaining the equivalence of the thermodynamics for systems characterized by
the same second virial coefficient. An outcome of our approach is the
possibility of counting the number of floppy modes (and their entropy).Comment: 4 pages, 4 figures accepted for publication on PR
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
String attractors and combinatorics on words
The notion of string attractor has recently been introduced in [Prezza, 2017] and studied in [Kempa and Prezza, 2018] to provide a unifying framework for known dictionary-based compressors. A string attractor for a word w = w[1]w[2] · · · w[n] is a subset Γ of the positions 1, . . ., n, such that all distinct factors of w have an occurrence crossing at least one of the elements of Γ. While finding the smallest string attractor for a word is a NP-complete problem, it has been proved in [Kempa and Prezza, 2018] that dictionary compressors can be interpreted as algorithms approximating the smallest string attractor for a given word. In this paper we explore the notion of string attractor from a combinatorial point of view, by focusing on several families of finite words. The results presented in the paper suggest that the notion of string attractor can be used to define new tools to investigate combinatorial properties of the words
Quasi-saddles as relevant points of the potential energy surface in the dynamics of supercooled liquids
The supercooled dynamics of a Lennard-Jones model liquid is numerically
investigated studying relevant points of the potential energy surface, i.e. the
minima of the square gradient of total potential energy . The main findings
are: ({\it i}) the number of negative curvatures of these sampled points
appears to extrapolate to zero at the mode coupling critical temperature ;
({\it ii}) the temperature behavior of has a close relationship with the
temperature behavior of the diffusivity; ({\it iii}) the potential energy
landscape shows an high regularity in the distances among the relevant points
and in their energy location. Finally we discuss a model of the landscape,
previously introduced by Madan and Keyes [J. Chem. Phys. {\bf 98}, 3342
(1993)], able to reproduce the previous findings.Comment: To be published in J. Chem. Phy
Mode-Coupling Theory of Colloids with Short-range Attractions
Within the framework of the mode-coupling theory of super-cooled liquids, we
investigate new phenomena in colloidal systems on approach to their glass
transitions. When the inter-particle potential contains an attractive part,
besides the usual repulsive hard core, two intersecting liquid-glass transition
lines appear, one of which extends to low densities, while the other one, at
high densities, shows a re-entrant behaviour. In the glassy region a new type
of transition appears between two different types of glasses. The complex
phenomenology can be described in terms of higher order glass transition
singularities. The various glass phases are characterised by means of their
viscoelastic properties. The glass driven by attractions has been associated to
particle gels, and the other glass is the well known repulsive colloidal glass.
These correspondences, in associations with the new predictions of glassy
behaviour mean that such phenomena may be expected in colloidal systems with,
for example, strong depletion or other short-ranged attractive potentials.Comment: 17 pages, 8 figure
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