6,744 research outputs found
Modeling of Covalent Bonding in Solids by Inversion of Cohesive Energy Curves
We provide a systematic test of empirical theories of covalent bonding in
solids using an exact procedure to invert ab initio cohesive energy curves. By
considering multiple structures of the same material, it is possible for the
first time to test competing angular functions, expose inconsistencies in the
basic assumption of a cluster expansion, and extract general features of
covalent bonding. We test our methods on silicon, and provide the direct
evidence that the Tersoff-type bond order formalism correctly describes
coordination dependence. For bond-bending forces, we obtain skewed angular
functions that favor small angles, unlike existing models. As a
proof-of-principle demonstration, we derive a Si interatomic potential which
exhibits comparable accuracy to existing models.Comment: 4 pages revtex (twocolumn, psfig), 3 figures. Title and some wording
(but no content) changed since original submission on 24 April 199
Branching, Capping, and Severing in Dynamic Actin Structures
Branched actin networks at the leading edge of a crawling cell evolve via
protein-regulated processes such as polymerization, depolymerization, capping,
branching, and severing. A formulation of these processes is presented and
analyzed to study steady-state network morphology. In bulk, we identify several
scaling regimes in severing and branching protein concentrations and find that
the coupling between severing and branching is optimally exploited for
conditions {\it in vivo}. Near the leading edge, we find qualitative agreement
with the {\it in vivo} morphology.Comment: 4 pages, 2 figure
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Parallel Mapper
The construction of Mapper has emerged in the last decade as a powerful and
effective topological data analysis tool that approximates and generalizes
other topological summaries, such as the Reeb graph, the contour tree, split,
and joint trees. In this paper, we study the parallel analysis of the
construction of Mapper. We give a provably correct parallel algorithm to
execute Mapper on multiple processors and discuss the performance results that
compare our approach to a reference sequential Mapper implementation. We report
the performance experiments that demonstrate the efficiency of our method
Collective vibrational states with fast iterative QRPA method
An iterative method we previously proposed to compute nuclear strength
functions is developed to allow it to accurately calculate properties of
individual nuclear states. The approach is based on the
quasi-particle-random-phase approximation (QRPA) and uses an iterative
non-hermitian Arnoldi diagonalization method where the QRPA matrix does not
have to be explicitly calculated and stored. The method gives substantial
advantages over conventional QRPA calculations with regards to the
computational cost. The method is used to calculate excitation energies and
decay rates of the lowest lying 2+ and 3- states in Pb, Sn, Ni and Ca isotopes
using three different Skyrme interactions and a separable gaussian pairing
force.Comment: 10 pages, 11 figure
Band-overlap metallization of BaS, BaSe and BaTe
The insulator-metal transition volumes for BaS, BaSe, and BaTe are calculated for the first time, using the self-consistent augmented spherical wave technique. The metallized transition volumes are smaller than those corresponding to the NaCl yields CsCl structural transitions, but, 10 to 15% larger than those obtained by the Herzfeld dielectric theory. The calculated equilibrium energy gaps in the NaCl structure underestimate the measured ones by 50 to 60%
Solid-Liquid Phase Diagrams for Binary Metallic Alloys: Adjustable Interatomic Potentials
We develop a new approach to determining LJ-EAM potentials for alloys and use
these to determine the solid-liquid phase diagrams for binary metallic alloys
using Kofke's Gibbs-Duhem integration technique combined with semigrand
canonical Monte Carlo simulations. We demonstrate that it is possible to
produce a wide-range of experimentally observed binary phase diagrams (with no
intermetallic phases) by reference to the atomic sizes and cohesive energies of
the two elemental materials. In some cases, it is useful to employ a single
adjustable parameter to adjust the phase diagram (we provided a good choice for
this free parameter). Next, we perform a systematic investigation of the effect
of relative atomic sizes and cohesive energies of the elements on the binary
phase diagrams. We then show that this approach leads to good agreement with
several experimental binary phase diagrams. The main benefit of this approach
is not the accurately reproduction of experimental phase diagrams, but rather
to provide a method by which material properties can be continuously changed in
simulations studies. This is one of the keys to the use of atomistic
simulations to understand mechanisms and properties in a manner not available
to experiment
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