563 research outputs found
The Marching 97: A History of the Finest Band East of All Points West
A history of Lehigh University\u27s Marching 97 marching band
Optimized Verlet-like algorithms for molecular dynamics simulations
New explicit velocity- and position-Verlet-like algorithms of the second
order are proposed to integrate the equations of motion in many-body systems.
The algorithms are derived on the basis of an extended decomposition scheme at
the presence of a free parameter. The nonzero value for this parameter is
obtained by reducing the influence of truncated terms to a minimum. As a
result, the new algorithms appear to be more efficient than the original Verlet
versions which correspond to a particular case when the introduced parameter is
equal to zero. Like the original versions, the proposed counterparts are
symplectic and time reversible, but lead to an improved accuracy in the
generated solutions at the same overall computational costs. The advantages of
the new algorithms are demonstrated in molecular dynamics simulations of a
Lennard-Jones fluid.Comment: 5 pages, 2 figures; submitted to Phys. Rev.
Algorithm for numerical integration of the rigid-body equations of motion
A new algorithm for numerical integration of the rigid-body equations of
motion is proposed. The algorithm uses the leapfrog scheme and the quantities
involved are angular velocities and orientational variables which can be
expressed in terms of either principal axes or quaternions. Due to specific
features of the algorithm, orthonormality and unit norms of the orientational
variables are integrals of motion, despite an approximate character of the
produced trajectories. It is shown that the method presented appears to be the
most efficient among all known algorithms of such a kind.Comment: 4 pages, 1 figur
On the construction of high-order force gradient algorithms for integration of motion in classical and quantum systems
A consequent approach is proposed to construct symplectic force-gradient
algorithms of arbitrarily high orders in the time step for precise integration
of motion in classical and quantum mechanics simulations. Within this approach
the basic algorithms are first derived up to the eighth order by direct
decompositions of exponential propagators and further collected using an
advanced composition scheme to obtain the algorithms of higher orders. Contrary
to the scheme by Chin and Kidwell [Phys. Rev. E 62, 8746 (2000)], where
high-order algorithms are introduced by standard iterations of a force-gradient
integrator of order four, the present method allows to reduce the total number
of expensive force and its gradient evaluations to a minimum. At the same time,
the precision of the integration increases significantly, especially with
increasing the order of the generated schemes. The algorithms are tested in
molecular dynamics and celestial mechanics simulations. It is shown, in
particular, that the efficiency of the new fourth-order-based algorithms is
better approximately in factors 5 to 1000 for orders 4 to 12, respectively. The
results corresponding to sixth- and eighth-order-based composition schemes are
also presented up to the sixteenth order. For orders 14 and 16, such highly
precise schemes, at considerably smaller computational costs, allow to reduce
unphysical deviations in the total energy up in 100 000 times with respect to
those of the standard fourth-order-based iteration approach.Comment: 23 pages, 2 figures; submitted to Phys. Rev.
Spontaneous alloying in binary metal microclusters - A molecular dynamics study -
Microcanonical molecular dynamics study of the spontaneous alloying(SA),
which is a manifestation of fast atomic diffusion in a nano-sized metal
cluster, is done in terms of a simple two dimensional binary Morse model.
Important features observed by Yasuda and Mori are well reproduced in our
simulation. The temperature dependence and size dependence of the SA phenomena
are extensively explored by examining long time dynamics. The dominant role of
negative heat of solution in completing the SA is also discussed. We point out
that a presence of melting surface induces the diffusion of core atoms even if
they are solid-like. In other words, the {\it surface melting} at substantially
low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.
Chemical Abundances of the Leo II Dwarf Galaxy
We use previously-published moderate-resolution spectra in combination with
stellar atmosphere models to derive the first measured chemical abundance
ratios in the Leo II dSph galaxy. We find that for spectra with SNR > 24, we
are able to measure abundances from weak Ti, Fe and Mg lines located near the
calcium infrared triplet (CaT). We also quantify and discuss discrepancies
between the metallicities measured from Fe I lines and those estimated from the
CaT features. We find that while the most metal-poor ([Fe/H] <-2.0]) Leo II
stars have Ca and Ti abundance ratios similar to those of Galactic globular
clusters, the more metal-rich stars show a gradual decline of Ti, Mg and Ca
abundance ratio with increasing metallicity. Finding these trends in this
distant and apparently dynamically stable dSph galaxy supports the hypothesis
that the slow chemical enrichment histories of the dSph galaxies is universal,
independent of any interaction with the Milky Way. Combining our spectroscopic
abundances with published broadband photometry and updated isochrones, we are
able to approximate stellar ages for our bright RGB stars to a relative
precision of 2-3 Gyr. While the derived age-metallicity relationship of Leo II
hints at some amount of slow enrichment, the data are still statistically
consistent with no enrichment over the history of Leo II.Comment: Accepted to A
Atomistic mechanisms for the ordered growth of Co nano-dots on Au(788): comparison of VT-STM experiments and multi-scaled calculations
Hetero-epitaxial growth on a strain-relief vicinal patterned substrate has
revealed unprecedented 2D long range ordered growth of uniform cobalt
nanostructures. The morphology of a Co sub-monolayer deposit on a Au(111)
reconstructed vicinal surface is analyzed by Variable Temperature Scanning
Tunneling Microscopy (VT-STM) experiments. A rectangular array of nano-dots
(3.8 nm x 7.2 nm) is found for a particularly large deposit temperature range
lying from 60 K to 300 K. Although the nanodot lattice is stable at room
temperature, this paper focus on the early stage of ordered nucleation and
growth at temperatures between 35 K and 480 K. The atomistic mechanisms leading
to the nanodots array are elucidated by comparing statistical analysis of
VT-STM images with multi-scaled numerical calculations combining both Molecular
Dynamics for the quantitative determination of the activation energies for the
atomic motion and the Kinetic Monte Carlo method for the simulations of the
mesoscopic time and scale evolution of the Co submonolayer
Exploring the Leo II dSph I.: The Variable Star Content
We present the first comprehensive catalogue of variable stars in the Leo II
dwarf spheroidal galaxy. We have identified 148 RR Lyrae type variables, of
which 140 were amenable to derivation of variability parameters with our data.
We have also confirmed the existence of four anomalous Cepheids as identified
in previous studies.
The average period of the RR Lyrae ab variables (0.62 days), the fraction of
c variables (0.24) and the minimum period of the RR Lyrae ab variables (0.51
days) all define Leo II as an "Oosterhoff intermediate" galaxy. We have used
the properties of these variables to derive a metallicity for Leo II of
approximately [Fe/H]=-1.9. The presence of longer period, higher amplitude RR
Lyrae variable implies a metallicity distribution that extends to as poor as
[Fe/H]=-2.3.
Leo II's location on the period-metallicity relation of clusters, like that
of other ``Ootershoff intermediate'' objects, falls between the Oosterhoff
Class I and Oosterhoff Class II clusters. The properties of the variable
populations of these objects are consistent with the idea that the Oosterhoff
"dichotomy" is a continuum. The gap between the classes seems to be explained
by the horizontal branch of Galactic globular clusters shifting away from the
instability strip at at intermediate metallicities. However, Leo II, as well as
other Oosterhoff intermediate objects, has a second parameter effect strong
enough to leave horizontal branch stars in the instability strip.Comment: 44 pages, 8 figures, 3 tables, accepted in Astronomical Journa
Atomic-scale modeling of the deformation of nanocrystalline metals
Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display
technologically interesting properties, such as dramatically increased
hardness, increasing with decreasing grain size. Due to the small grain size,
direct atomic-scale simulations of plastic deformation of these materials are
possible, as such a polycrystalline system can be modeled with the
computational resources available today.
We present molecular dynamics simulations of nanocrystalline copper with
grain sizes up to 13 nm. Two different deformation mechanisms are active, one
is deformation through the motion of dislocations, the other is sliding in the
grain boundaries. At the grain sizes studied here the latter dominates, leading
to a softening as the grain size is reduced. This implies that there is an
``optimal'' grain size, where the hardness is maximal.
Since the grain boundaries participate actively in the deformation, it is
interesting to study the effects of introducing impurity atoms in the grain
boundaries. We study how silver atoms in the grain boundaries influence the
mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in
Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see
http://www.fysik.dtu.dk/~schiotz/publist.htm
Why are MD simulated protein folding times wrong?
The question of significant deviations of protein folding times simulated using molecular dynamics from experimental values is investigated. It is shown that in the framework of Markov State Model (MSM) describing the conformational dynamics of peptides and proteins, the folding time is very sensitive to the simulation model parameters, such as forcefield and temperature. Using two peptides as examples, we show that the deviations in the folding times can reach an order of magnitude for modest variations of the molecular model. We, therefore, conclude that the folding rate values obtained in molecular dynamics simulations have to be treated with care
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