91 research outputs found
Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: a theoretical and experimental study
We study the underpotential deposition of Cu on single-crystal Au(111)
electrodes in sulfate-containing electrolytes by a combination of computational
statistical-mechanics based lattice-gas modeling and experiments. The
experimental methods are in situ cyclic voltammetry and coulometry and ex situ
Auger electron spectroscopy and low-energy electron diffraction. The
experimentally obtained voltammetric current and charge densities and adsorbate
coverages are compared with the predictions of a two-component lattice-gas
model for the coadsorption of Cu and sulfate. This model includes effective,
lateral interactions out to fourth-nearest neighbors. Using group-theoretical
ground-state calculations and Monte Carlo simulations, we estimate effective
electrovalences and lateral adsorbate--adsorbate interactions so as to obtain
overall agreement with experiments, including both our own and those of other
groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting
of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode
potentials, delimited by phase transitions at both higher and lower potentials.
Our approach provides estimates of the effective electrovalences and lateral
interaction energies, which cannot yet be calculated by first-principles
methods.Comment: 36 pages, 14 Postscript figures are in uufiles for
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Beyond Finite Size Scaling in Solidification Simulations
Although computer simulation has played a central role in the study of nucleation and growth since the earliest molecular dynamics simulations almost 50 years ago, confusion surrounding the effect of finite size on such simulations have limited their applicability. Modeling solidification in molten tantalum on the BlueGene/L computer, we report here on the first atomistic simulation of solidification that verifies independence from finite size effects during the entire nucleation and growth process, up to the onset of coarsening. We show that finite size scaling theory explains the observed maximal grain sizes for systems up to about 8,000,000 atoms. For larger simulations, a cross-over from finite size scaling to more physical size-independent behavior is observed
Atoms in the Surf: Molecular Dynamics Simulation of the Kelvin-Helmholtz Instability using 9 Billion Atoms
We present a fluid dynamics video showing the results of a 9-billion atom
molecular dynamics simulation of complex fluid flow in molten copper and
aluminum. Starting with an atomically flat interface, a shear is imposed along
the copper-aluminum interface and random atomic fluctuations seed the formation
of vortices. These vortices grow due to the Kelvin-Helmholtz instability. The
resulting vortical structures are beautifully intricate, decorated with
secondary instabilities and complex mixing phenomena. This work performed under
the auspices of the U.S. Department of Energy by Lawrence Livermore National
Laboratory under Contract DE-AC52-07NA27344.Comment: Description of video submitted to APS DFD Gallery of Fluid Motion
200
Liquid-liquid equilibrium for monodisperse spherical particles
A system of identical particles interacting through an isotropic potential
that allows for two preferred interparticle distances is numerically studied.
When the parameters of the interaction potential are adequately chosen, the
system exhibits coexistence between two different liquid phases (in addition to
the usual liquid-gas coexistence). It is shown that this coexistence can occur
at equilibrium, namely, in the region where the liquid is thermodynamically
stable.Comment: 6 pages, 8 figures. Published versio
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Quantum-Based Atomistic Simulation of Metals at Extreme Conditions
First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for bridging the quantum-atomistic gap from density-functional quantum mechanics to large scale atomistic simulation in metals and alloys. In directionally-bonded bcc transition metals, advanced generation model GPT or MGPT potentials based on canonical d bands have been developed for Ta, Mo and V and successfully applied to a wide range of thermodynamic and mechanical properties at both ambient and extreme conditions of pressure and temperature, including high-pressure phase transitions, multiphase equation of state; melting and solidification; thermoelasticity; and the atomistic simulation of point defects, dislocations and grain boundaries needed for the multiscale modeling of plasticity and strength. Recent algorithm improvements have also allowed an MGPT implementation beyond canonical bands to achieve increased accuracy, extension to f-electron actinide metals, and high computational speed. A further advance in progress is the development temperature-dependent MGPT potentials that subsume electron-thermal contributions to high-temperature properties
Theory for the ultrafast ablation of graphite films
The physical mechanisms for damage formation in graphite films induced by
femtosecond laser pulses are analyzed using a microscopic electronic theory. We
describe the nonequilibrium dynamics of electrons and lattice by performing
molecular dynamics simulations on time-dependent potential energy surfaces. We
show that graphite has the unique property of exhibiting two distinct laser
induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we
find nonequilibrium melting followed by fast evaporation. For low intensities
above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of
intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re
Molecular Dynamics Simulations of Temperature Equilibration in Dense Hydrogen
The temperature equilibration rate in dense hydrogen (for both T_{i}>T_{e}
and T_i<T_e) has been calculated with molecular dynamics simulations for
temperatures between 10 and 600 eV and densities between 10^{20}/cc to
10^{24}/cc. Careful attention has been devoted to convergence of the
simulations, including the role of semiclassical potentials. We find that for
Coulomb logarithms L>1, a model by Gericke-Murillo-Schlanges (GMS) [Gericke et
al., PRE 65, 036418 (2002)] based on a T-matrix method and the approach by
Brown-Preston-Singleton [Brown et al., Phys. Rep. 410, 237 (2005)] agrees with
the simulation data to within the error bars of the simulation. For smaller
Coulomb logarithms, the GMS model is consistent with the simulation results.
Landau-Spitzer models are consistent with the simulation data for L>4
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Quantum-based Atomistic Simulation of Transition Metals
First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in d-electron transition metals within density-functional quantum mechanics. In mid-period bcc metals, where multi-ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature. Recent algorithm improvements have also led to a more general matrix representation of MGPT beyond canonical bands allowing increased accuracy and extension to f-electron actinide metals, an order of magnitude increase in computational speed, and the current development of temperature-dependent potentials
Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly
We investigate the phase behavior of a single-component system in 3
dimensions with spherically-symmetric, pairwise-additive, soft-core
interactions with an attractive well at a long distance, a repulsive soft-core
shoulder at an intermediate distance, and a hard-core repulsion at a short
distance, similar to potentials used to describe liquid systems such as
colloids, protein solutions, or liquid metals. We showed [Nature {\bf 409}, 692
(2001)] that, even with no evidences of the density anomaly, the phase diagram
has two first-order fluid-fluid phase transitions, one ending in a
gas--low-density liquid (LDL) critical point, and the other in a
gas--high-density liquid (HDL) critical point, with a LDL-HDL phase transition
at low temperatures. Here we use integral equation calculations to explore the
3-parameter space of the soft-core potential and we perform molecular dynamics
simulations in the interesting region of parameters. For the equilibrium phase
diagram we analyze the structure of the crystal phase and find that, within the
considered range of densities, the structure is independent of the density.
Then, we analyze in detail the fluid metastable phases and, by explicit
thermodynamic calculation in the supercooled phase, we show the absence of the
density anomaly. We suggest that this absence is related to the presence of
only one stable crystal structure.Comment: 15 pages, 21 figure
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