77 research outputs found
Saddle Points and Dynamics of Lennard-Jones Clusters, Solids and Supercooled Liquids
The properties of higher-index saddle points have been invoked in recent
theories of the dynamics of supercooled liquids. Here we examine in detail a
mapping of configurations to saddle points using minimization of , which has been used in previous work to support these theories. The
examples we consider are a two-dimensional model energy surface and binary
Lennard-Jones liquids and solids. A shortcoming of the mapping is its failure
to divide the potential energy surface into basins of attraction surrounding
saddle points, because there are many minima of that do not
correspond to stationary points of the potential energy. In fact, most liquid
configurations are mapped to such points for the system we consider. We
therefore develop an alternative route to investigate higher-index saddle
points and obtain near complete distributions of saddles for small
Lennard-Jones clusters. The distribution of the number of stationary points as
a function of the index is found to be Gaussian, and the average energy
increases linearly with saddle point index in agreement with previous results
for bulk systems.Comment: 14 pages, 7 figure
Surface structure in simple liquid metals. An orbital free first principles study
Molecular dynamics simulations of the liquid-vapour interfaces in simple
sp-bonded liquid metals have been performed using first principles methods.
Results are presented for liquid Li, Na, K, Rb, Cs, Mg, Ba, Al, Tl, and Si at
thermodynamic conditions near their respective triple points, for samples of
2000 particles in a slab geometry. The longitudinal ionic density profiles
exhibit a pronounced stratification extending several atomic diameters into the
bulk, which is a feature already experimentally observed in liquid K, Ga, In,
Sn and Hg. The wavelength of the ionic oscillations shows a good scaling with
the radii of the associated Wigner-Seitz spheres. The structural rearrangements
at the interface are analyzed in terms of the transverse pair correlation
function, the coordination number and the bond-angle distribution between
nearest neighbors. The valence electronic density profile also shows (weaker)
oscillations whose phase, with respect to those of the ionic profile, changes
from opposite phase in the alkalis to almost in-phase for Si.Comment: 16 pages, 18 figures, 5 tables. Submitted to Phys. Rev.
Tendency to occupy a statistically dominant spatial state of the flow as a driving force for turbulent transition
A simple analytical model for a turbulent flow is proposed, which considers
the flow as a collection of localized spatial structures that are composed of
elementary "cells" in which the state of the particles (atoms or molecules) is
uncertain. The Reynolds number is associated with the ratio between the total
phase volume for the system and that for the elementary cell. Calculating the
statistical weights of the collections of the localized structures, it is shown
that as the Reynolds number increases, the elementary cells group into the
localized structures, which successfully explains the onset of turbulence and
some other characteristic properties of turbulent flows. It is also shown that
the basic assumptions underlying the model are involved in the derivation of
the Navier-Stokes equation, which suggests that the driving force for the
turbulent transition described with the hydrodynamic equations is essentially
the same as in the present model, i.e. the tendency of the system to occupy a
statistically dominant state plays a key role. The instability of the flow can
then be a mechanism to initiate the structural rearrangement of the flow to
find this state.Comment: 7 pages, 4 figures, and Supplementary Material (2 pages, 3 figures),
to be submitted to "Foundations of Physics". arXiv admin note: substantial
text overlap with arXiv:1102.515
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Atomic layering at the liquid silicon surface: a first- principles simulation
We simulate the liquid silicon surface with first-principles molecular
dynamics in a slab geometry. We find that the atom-density profile presents a
pronounced layering, similar to those observed in low-temperature liquid metals
like Ga and Hg. The depth-dependent pair correlation function shows that the
effect originates from directional bonding of Si atoms at the surface, and
propagates into the bulk. The layering has no major effects in the electronic
and dynamical properties of the system, that are very similar to those of bulk
liquid Si. To our knowledge, this is the first study of a liquid surface by
first-principles molecular dynamics.Comment: 4 pages, 4 figures, submitted to PR
Exploring the Free Energy Landscape: From Dynamics to Networks and Back
The knowledge of the Free Energy Landscape topology is the essential key to
understand many biochemical processes. The determination of the conformers of a
protein and their basins of attraction takes a central role for studying
molecular isomerization reactions. In this work, we present a novel framework
to unveil the features of a Free Energy Landscape answering questions such as
how many meta-stable conformers are, how the hierarchical relationship among
them is, or what the structure and kinetics of the transition paths are.
Exploring the landscape by molecular dynamics simulations, the microscopic data
of the trajectory are encoded into a Conformational Markov Network. The
structure of this graph reveals the regions of the conformational space
corresponding to the basins of attraction. In addition, handling the
Conformational Markov Network, relevant kinetic magnitudes as dwell times or
rate constants, and the hierarchical relationship among basins, complete the
global picture of the landscape. We show the power of the analysis studying a
toy model of a funnel-like potential and computing efficiently the conformers
of a short peptide, the dialanine, paving the way to a systematic study of the
Free Energy Landscape in large peptides.Comment: PLoS Computational Biology (in press
Two-pulse stimulated echo in magnets
The results of experimental study of two-pulse stimulated echo in
ferromagnets of two types are presented. Ferromagnet Co and half-metal Co MnSi
2, in which a single-pulse echo formed by the distortion mechanism of the
fronts of exciting pulse is also observed, are classified among the first type.
Lithium ferrite and intermetal compound MnSb characterized by the absence of
single-pulse echo in them - belong to the second type. For signals of two-pulse
stimulated echo in the materials of the first type a short time and a long time
of relaxations are observed. The short time is about the order of value shorter
less than the spin-spin relaxation time. The long time is close to the
transverse relaxation time of single-pulse echo formed by the distortion
mechanism. The mechanisms that provide the possible interpretations of the
peculiarities of the processes of nuclear magnetic relaxation are discussed.Comment: 10 pages, 7 figure
Density Distribution in the Liquid Hg-Sapphire Interface
We present the results of a computer simulation study of the liquid density
distribution normal to the interface between liquid Hg and the reconstructed
(0001) face of sapphire. The simulations are based on an extension of the
self-consistent quantum Monte Carlo scheme previously used to study the
structure of the liquid metal-vapor interface. The calculated density
distribution is in very good agreement with that inferred from the recent
experimental data of Tamam et al (J. Phys. Chem. Lett. 1, 1041-1045 (2010)). We
conclude that, to account for the difference in structure between the liquid
Hg-vapor and liquid-Hg-reconstructed (0001) Al2O3 interfaces, it is not
necessary assume there is charge transfer from the Hg to the Al2O3. Rather, the
available experimental data are adequately reproduced when the van der Waals
interactions of the Al and O atoms with Hg atoms and the exclusion of electron
density from Al2O3 via repulsion of the electrons from the closed shells of the
ions in the solid are accounted for.Comment: 26 pages, 11 figure
Application of 3D Zernike descriptors to shape-based ligand similarity searching
Background: The identification of promising drug leads from a large database of compounds is an important step in the preliminary stages of drug design. Although shape is known to play a key role in the molecular recognition process, its application to virtual screening poses significant hurdles both in terms of the encoding scheme and speed. Results: In this study, we have examined the efficacy of the alignment independent three-dimensional Zernike descriptor (3DZD) for fast shape based similarity searching. Performance of this approach was compared with several other methods including the statistical moments based ultrafast shape recognition scheme (USR) and SIMCOMP, a graph matching algorithm that compares atom environments. Three benchmark datasets are used to thoroughly test the methods in terms of their ability for molecular classification, retrieval rate, and performance under the situation that simulates actual virtual screening tasks over a large pharmaceutical database. The 3DZD performed better than or comparable to the other methods examined, depending on the datasets and evaluation metrics used. Reasons for the success and the failure of the shape based methods for specific cases are investigated. Based on the results for the three datasets, general conclusions are drawn with regard to their efficiency and applicability
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