49,039 research outputs found
Different Melting Behavior in Pentane and Heptane Monolayers on Graphite; Molecular Dynamics Simulations
Molecular dynamics simulations are utilized to study the melting transition
in pentane (C5H12) and heptane (C7H16), physisorbed onto the basal plane of
graphite at near-monolayer coverages. Through use of the newest, optimized
version of the anisotropic united-atom model (AUA4) to simulate both systems at
two separate coverages, this study provides evidence that the melting
transition for pentane and heptane monolayers are significantly different.
Specifically, this study proposes a very rapid transition from the solid
crystalline rectangular-centered (RC) phase to a fluid phase in pentane
monolayers, whereas heptane monolayers exhibit a slower transition that
involves a more gradual loss of RC order in the solid-fluid phase transition.
Through a study of the melting behavior, encompassing variations where the
formation of gauche defects in the alkyl chains are eliminated, this study
proposes that this gradual melting behavior for heptane monolayers is a result
of less orientational mobility of the heptane molecules in the solid RC phase,
as compared to the pentane molecules. This idea is supported through a study of
a nonane monolayer, which gives the gradual melting signature that heptane
monolayers also seem to indicate. The results of this work are compared to
previous experiment over pentane and heptane monolayers, and are found to be in
good agreement
Spin dynamics for the Lebwohl-Lasher model
A spin dynamics algorithm, combining checkerboard updating and a rotation algorithm based on the local second-rank ordering field, is developed for the Lebwohl-Lasher model of liquid crystals. The method is shown to conserve energy well and to generate simulation averages that are consistent with those obtained by Monte Carlo simulation. However, care must be taken to avoid the undesirable effects of director rotation, and a method for doing this is proposed
Well-defined silica-supported olefin metathesis catalysts
Two triethoxysilyl-functionalized N-heterocyclic carbene ligands have been synthesized and used to prepare the corresponding second-generation ruthenium olefin metathesis catalysts. These complexes were then grafted onto silica gel, and the resulting materials were efficient heterogeneous catalysts for a number of metathesis reactions. The solid-supported catalysts were shown to be recyclable over a number of reaction cycles, and no detectable levels of ruthenium were observed in reaction filtrates (ruthenium concentration of filtrate <5 ppb)
Ab initio mass tensor molecular dynamics
Mass tensor molecular dynamics was first introduced by Bennett [J. Comput.
Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of
generalized atomic masses. Here, we show how to apply this method to ab initio
molecular dynamics simulations with minimal computational overhead. Test
calculations on liquid water show a threefold reduction in computational effort
without making the fixed geometry approximation. We also present a simple
recipe for estimating the optimal atomic masses using only the first
derivatives of the potential energy.Comment: 19 pages, 5 figure
New Green-Kubo formulas for transport coefficients in hard sphere-, Langevin fluids and the likes
We present generalized Green-Kubo expressions for thermal transport
coefficients in non-conservative fluid-type systems, of the generic form,
+\int^\infty_0 dt V^{-1} \av{I_\epsilon \exp(t {\cal L})
I}_0 where is a pseudo-streaming operator. It consists of a
sum of an instantaneous transport coefficient , and a time integral
over a time correlation function in a state of thermal equilibrium between a
current and its conjugate current . This formula with
and covers vastly different systems,
such as strongly repulsive elastic interactions in hard sphere fluids, weakly
interacting Langevin fluids with dissipative and stochastic interactions
satisfying detailed balance conditions, and "the likes", defined in the text.
For conservative systems the results reduce to the standard formulas.Comment: 7 pages, no figures. Version 2: changes in the text and references
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Collective motion of binary self-propelled particle mixtures
In this study, we investigate the phenomenon of collective motion in binary
mixtures of self-propelled particles. We consider two particle species, each of
which consisting of pointlike objects that propel with a velocity of constant
magnitude. Within each species, the particles try to achieve polar alignment of
their velocity vectors, whereas we analyze the cases of preferred polar,
antiparallel, as well as perpendicular alignment between particles of different
species. Our focus is on the effect that the interplay between the two species
has on the threshold densities for the onset of collective motion and on the
nature of the solutions above onset. For this purpose, we start from suitable
Langevin equations in the particle picture, from which we derive mean field
equations of the Fokker-Planck type and finally macroscopic continuum field
equations. We perform particle simulations of the Langevin equations, linear
stability analyses of the Fokker-Planck and macroscopic continuum equations,
and we numerically solve the Fokker-Planck equations. Both, spatially
homogeneous and inhomogeneous solutions are investigated, where the latter
correspond to stripe-like flocks of collectively moving particles. In general,
the interaction between the two species reduces the threshold density for the
onset of collective motion of each species. However, this interaction also
reduces the spatial organization in the stripe-like flocks. The most
interesting behavior is found for the case of preferred perpendicular alignment
between different species. There, a competition between polar and truly nematic
orientational ordering of the velocity vectors takes place within each particle
species. Finally, depending on the alignment rule for particles of different
species and within certain ranges of particle densities, identical and inverted
spatial density profiles can be found for the two particle species.Comment: 16 pages, 10 figure
NVU dynamics. III. Simulating molecules at constant potential energy
This is the final paper in a series that introduces geodesic molecular
dynamics at constant potential energy. This dynamics is entitled NVU dynamics
in analogy to standard energy-conserving Newtonian NVE dynamics. In the first
two papers [Ingebrigtsen et al., J. Chem. Phys. 135, 104101 (2011); ibid,
104102 (2011)], a numerical algorithm for simulating geodesic motion of atomic
systems was developed and tested against standard algorithms. The conclusion
was that the NVU algorithm has the same desirable properties as the Verlet
algorithm for Newtonian NVE dynamics, i.e., it is time-reversible and
symplectic. Additionally, it was concluded that NVU dynamics becomes equivalent
to NVE dynamics in the thermodynamic limit. In this paper, the NVU algorithm
for atomic systems is extended to be able to simulate geodesic motion of
molecules at constant potential energy. We derive an algorithm for simulating
rigid bonds and test this algorithm on three different systems: an asymmetric
dumbbell model, Lewis-Wahnstrom OTP, and rigid SPC/E water. The rigid bonds
introduce additional constraints beyond that of constant potential energy for
atomic systems. The rigid-bond NVU algorithm conserves potential energy, bond
lengths, and step length for indefinitely long runs. The quantities probed in
simulations give results identical to those of Nose-Hoover NVT dynamics. Since
Nose-Hoover NVT dynamics is known to give results equivalent to those of NVE
dynamics, the latter results show that NVU dynamics becomes equivalent to NVE
dynamics in the thermodynamic limit also for molecular systems.Comment: 14 pages, 12 figure
Six New Galactic Orbits of Globular Clusters in a Milky-Way-Like Galaxy
Absolute proper motions for six new globular clusters have recently been
determined. This motivated us to obtain the Galactic orbits of these six
clusters both in an axisymmetric Galactic potential and in a barred potential,
such as the one of our Galaxy. Orbits are also obtained for a Galactic
potential that includes spiral arms. The orbital characteristics are compared
and discussed for these three cases. Tidal radii and destruction rates are also
computed and discussed.Comment: 29 pages, 11 figures. Accepted for publication in Ap
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