3,250 research outputs found
INCORPORATION OF QUANTUM STATISTICAL FEATURES IN MOLECULAR DYNAMICS
We formulate a method for incorporating quantum fluctuations into molecular-
dynamics simulations of many-body systems, such as those employed for energetic
nuclear collision processes. Based on Fermi's Golden Rule, we allow spontaneous
transitions to occur between the wave packets which are not energy eigenstates.
The ensuing diffusive evolution in the space of the wave packet parameters
exhibits appealing physical properties, including relaxation towards quantum-
statistical equilibrium.Comment: 8 latex pages + 1 uuencoded ps figur
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding
Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
Detailed ground study of 8 - 13 micron infrared imagery, Carrizo Plains, California
Infrared imagery study to determine dark thermal anomalies associated with San Andreas fault trac
The ultra high resolution XUV spectroheliograph: An attached payload for the Space Station Freedom
The principle goal of the ultra high resolution XUV spectroheliograph (UHRXS) is to improve the ability to identify and understand the fundamental physical processes that shape the structure and dynamics of the solar chromosphere and corona. The ability of the UHRXS imaging telescope and spectrographs to resolve fine scale structures over a broad wavelength (and hence temperature) range is critical to this mission. The scientific objectives and instrumental capabilities of the UHRXS investigation are reviewed before proceeding to a discussion of the expected performance of the UHRXS observatory
Stability of Uniform Shear Flow
The stability of idealized shear flow at long wavelengths is studied in
detail. A hydrodynamic analysis at the level of the Navier-Stokes equation for
small shear rates is given to identify the origin and universality of an
instability at any finite shear rate for sufficiently long wavelength
perturbations. The analysis is extended to larger shear rates using a low
density model kinetic equation. Direct Monte Carlo Simulation of this equation
is computed with a hydrodynamic description including non Newtonian rheological
effects. The hydrodynamic description of the instability is in good agreement
with the direct Monte Carlo simulation for , where is the mean
free time. Longer time simulations up to are used to identify the
asymptotic state as a spatially non-uniform quasi-stationary state. Finally,
preliminary results from molecular dynamics simulation showing the instability
are presented and discussed.Comment: 25 pages, 9 figures (Fig.8 is available on request) RevTeX, submitted
to Phys. Rev.
Log-periodic drift oscillations in self-similar billiards
We study a particle moving at unit speed in a self-similar Lorentz billiard
channel; the latter consists of an infinite sequence of cells which are
identical in shape but growing exponentially in size, from left to right. We
present numerical computation of the drift term in this system and establish
the logarithmic periodicity of the corrections to the average drift
Nonequilibrium Microscopic Distribution of Thermal Current in Particle Systems
A nonequilibrium distribution function of microscopic thermal current is
studied by a direct numerical simulation in a thermal conducting steady state
of particle systems. Two characteristic temperatures of the thermal current are
investigated on the basis of the distribution. It is confirmed that the
temperature depends on the current direction; Parallel temperature to the
heat-flux is higher than antiparallel one. The difference between the parallel
temperature and the antiparallel one is proportional to a macroscopic
temperature gradient.Comment: 4 page
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