321 research outputs found
A lattice Boltzmann study of reactive microflows
The role of geometrical micro-barriers on the conversion efficiency of
reactive flows in narrow three-dimensional channels of millimetric size is
investigated. Using a Lattice-Boltzmann-Lax-Wendroff code, we show that
micro-barriers have an appreciable effect on the effective reaction efficiency
of the device. If extrapolated to macroscopic scales, these effects can result
in a sizeable increase of the overall reaction efficiency.Comment: 5 pages, 7 figure
The Amorphous-Crystal Interface in Silicon: a Tight-Binding Simulation
The structural features of the interface between the cystalline and amorphous
phases of Si solid are studied in simulations based on a combination of
empirical interatomic potentials and a nonorthogonal tight-binding model. The
tight-binding Hamiltonian was created and tested for the types of structures
and distortions anticipated to occur at this interface. The simulations
indicate the presence of a number of interesting features near the interface.
The features that may lead to crystallization upon heating include chains
with some defects, most prominently dimers similar to those on the Si(001) 2x1
reconstructed free surface. Within the amorphous region order is lost over very
short distances. By examining six different samples with two interfaces each,
we find the energy of the amorphous-crystal interface to be 0.49 +/- 0.05 J/m^2Comment: Submitted to Phys. Rev.
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
Reduction of Activation Energy Barrier of Stone-Wales Transformation in Endohedral Metallofullerenes
We examine effects of encapsulated metal atoms inside a C molecule on
the activation energy barrier to the Stone-Wales transformation using {\it ab
initio} calculations. The encapsulated metal atoms we study are K, Ca and La
which nominally donate one, two and three electrons to the C cage,
respectively. We find that isomerization of the endohedral metallofullerene via
the Stone-Wales transformation can occur more easily than that of the empty
fullerene owing to the charge transfer. When K, Ca and La atoms are
encapsulated inside the fullerene, the activation energy barriers are lowered
by 0.30, 0.55 and 0.80 eV, respectively compared with that of the empty
C (7.16 eV). The lower activation energy barrier of the Stone-Wales
transformation implies the higher probability of isomerization and coalescence
of metallofullerenes, which require a series of Stone-Wales transformations.Comment: 13 pages, 3 figures, 1 tabl
From Electrons to Finite Elements: A Concurrent Multiscale Approach for Metals
We present a multiscale modeling approach that concurrently couples quantum
mechanical, classical atomistic and continuum mechanics simulations in a
unified fashion for metals. This approach is particular useful for systems
where chemical interactions in a small region can affect the macroscopic
properties of a material. We discuss how the coupling across different scales
can be accomplished efficiently, and we apply the method to multiscale
simulations of an edge dislocation in aluminum in the absence and presence of H
impurities.Comment: 4 page
Multiscale model of electronic behavior and localization in stretched dry DNA
When the DNA double helix is subjected to external forces it can stretch elastically to elongations reaching 100% of its natural length. These distortions, imposed at the mesoscopic or macroscopic scales, have a dramatic effect on electronic properties at the atomic scale and on electrical transport along DNA. Accordingly, a multiscale approach is necessary to capture the electronic behavior of the stretched DNA helix. To construct such a model, we begin with accurate density-functional-theory calculations for electronic states in DNA bases and base pairs in various relative configurations encountered in the equilibrium and stretched forms. These results are complemented by semi-empirical quantum mechanical calculations for the states of a small size [18 base pair poly(CG)–poly(CG)] dry, neutral DNA sequence, using previously published models for stretched DNA. The calculated electronic states are then used to parametrize an effective tight-binding model that can describe electron hopping in the presence of environmental effects, such as the presence of stray water molecules on the backbone or structural features of the substrate. These effects introduce disorder in the model hamiltonian which leads to electron localization. The localization length is smaller by several orders of magnitude in stretched DNA relative to that in the unstretched structure
Chemical efficiency of reactive microflows with heterogeneus catalysis: a lattice Boltzmann study
We investigate the effects of geometrical micro-irregularities on the
conversion efficiency of reactive flows in narrow channels of millimetric size.
Three-dimensional simulations, based upon a Lattice-Boltzmann-Lax-Wendroff
code, indicate that periodic micro-barriers may have an appreciable effect on
the effective reaction efficiency of the device. Once extrapolated to
macroscopic scales, these effects can result in a sizeable increase of the
overall reaction efficiency.Comment: 12 pages, 12 figure
Electronic structure of the (111) and (-1-1-1) surfaces of cubic BN: A local-density-functional ab initio study
We present ab initio local-density-functional electronic structure
calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically
stable reconstructions, namely the N adatom, N3 triangle models on the (111),
the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are
investigated. Band structure and properties of the surface states are discussed
in detail.Comment: 8 pages, 12 figure
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