142,987 research outputs found
Radiation Magnetohydrodynamics Simulation of Proto-Stellar Collapse: Two-Component Molecular Outflow
We perform a three-dimensional nested-grid radiation magneto-hydrodynamics
(RMHD) simulation with self-gravity to study the early phase of the low-mass
star formation process from a rotating molecular cloud core to a first
adiabatic core just before the second collapse begins. Radiation transfer is
handled with the flux-limited diffusion approximation, operator-splitting and
implicit time-integrator. In the RMHD simulation, the outer region of the first
core attains a higher entropy and the size of first core is larger than that in
the magnetohydrodynamics simulations with the barotropic approximation. Bipolar
molecular outflow consisting of two components is driven by magnetic Lorentz
force via different mechanisms, and shock heating by the outflow is observed.
Using the RMHD simulation we can predict and interpret the observed properties
of star-forming clouds, first cores and outflows with millimeter/submillimeter
radio interferometers, especially the Atacama Large Millimeter/submillimeter
Array (ALMA).Comment: 13 pages, 5 figures, submitted to ApJ
Grid computing and molecular simulations: the vision of the eMinerals Project
This paper discusses a number of aspects of using grid computing methods in support of molecular simulations, with examples drawn from the eMinerals project. A number of components for a useful grid infrastructure are discussed, including the integration of compute and data grids, automatic metadata capture from simulation studies, interoperability of data between simulation codes, management of data and data accessibility, management of jobs and workflow, and tools to support collaboration. Use of a grid infrastructure also brings certain challenges, which are discussed. These include making use of boundless computing resources, the necessary changes, and the need to be able to manage experimentation
Fourier Acceleration of Langevin Molecular Dynamics
Fourier acceleration has been successfully applied to the simulation of
lattice field theories for more than a decade. In this paper, we extend the
method to the dynamics of discrete particles moving in continuum. Although our
method is based on a mapping of the particles' dynamics to a regular grid so
that discrete Fourier transforms may be taken, it should be emphasized that the
introduction of the grid is a purely algorithmic device and that no smoothing,
coarse-graining or mean-field approximations are made. The method thus can be
applied to the equations of motion of molecular dynamics (MD), or its Langevin
or Brownian variants. For example, in Langevin MD simulations our acceleration
technique permits a straightforward spectral decomposition of forces so that
the long-wavelength modes are integrated with a longer time step, thereby
reducing the time required to reach equilibrium or to decorrelate the system in
equilibrium. Speedup factors of up to 30 are observed relative to pure
(unaccelerated) Langevin MD. As with acceleration of critical lattice models,
even further gains relative to the unaccelerated method are expected for larger
systems. Preliminary results for Fourier-accelerated molecular dynamics are
presented in order to illustrate the basic concepts. Possible extensions of the
method and further lines of research are discussed.Comment: 11 pages, two illustrations included using graphic
DOF phase separation of the Lennard-Jones fcc(111) surface
Recent lattice model calculations have suggested that a full-layered crystal
surface may undergo, under canonical (particle-conserving) conditions, a
preroughening-driven two-dimensional phase separation into two disordered flat
(DOF) regions, of opposite order parameter. We have carried out extensive
classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111)
surface, to check whether these predictions are relevant or not for a realistic
continuous system. Very long simulation times, a grid of temperatures from
(2/3)Tm to Tm, and unusually large system sizes are employed to ensure full
equilibrium and good statistics. By examining layer-by-layer occupancies,
height fluctuations, sublattice order parameter and X-ray structure factors, we
find a clear anomaly at ~0.83Tm. The anomaly is distinct from roughening (whose
incipiency is also detected at ~0.94Tm), and is seen to be consistent with the
preroughening plus phase separation scenario.Comment: REVTeX, 8 pages, 4 figures; new figure showing simulation snapshots
added; reference updated and other minor change
A Thermodynamically-Consistent Non-Ideal Stochastic Hard-Sphere Fluid
A grid-free variant of the Direct Simulation Monte Carlo (DSMC) method is
proposed, named the Isotropic DSMC (I-DSMC) method, that is suitable for
simulating dense fluid flows at molecular scales. The I-DSMC algorithm
eliminates all grid artifacts from the traditional DSMC algorithm; it is
Galilean invariant and microscopically isotropic. The stochastic collision
rules in I-DSMC are modified to yield a non-ideal structure factor that gives
consistent compressibility, as first proposed in [Phys. Rev. Lett. 101:075902
(2008)]. The resulting Stochastic Hard Sphere Dynamics (SHSD) fluid is
empirically shown to be thermodynamically identical to a deterministic
Hamiltonian system of penetrable spheres interacting with a linear core pair
potential, well-described by the hypernetted chain (HNC) approximation. We
apply a stochastic Enskog kinetic theory for the SHSD fluid to obtain estimates
for the transport coefficients that are in excellent agreement with particle
simulations over a wide range of densities and collision rates. The fluctuating
hydrodynamic behavior of the SHSD fluid is verified by comparing its dynamic
structure factor against theory based on the Landau-Lifshitz Navier-Stokes
equations. We also study the Brownian motion of a nano-particle suspended in an
SHSD fluid and find a long-time power-law tail in its velocity autocorrelation
function consistent with hydrodynamic theory and molecular dynamics
calculations.Comment: 30 pages, revision adding some clarifications and a new figure. See
also arXiv:0803.035
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