2,718 research outputs found
Molecular dynamics of cleavage and flake formation during the interaction of a graphite surface with a rigid nanoasperity
Computer experiments concerning interactions between a graphite surface and
the rigid pyramidal nanoasperity of a friction force microscope tip when it is
brought close to and retracted from the graphitic sample are presented.
Covalent atomic bonds in graphene layers are described using a Brenner
potential and tip-carbon forces are derived from the Lennard-Jones potential.
For interlayer interactions a registry-dependent potential with local normals
is used. The behavior of the system is investigated under conditions of
different magnitudes of tip-sample interaction and indentation rates. Strong
forces between the nanoasperity and carbon atoms facilitate the cleavage of the
graphite surface. Exfoliation, i. e. total removal of the upper graphitic
layer, is observed when a highly adhesive tip is moved relative to the surface
at low rates, while high rates cause the formation of a small flake attached to
the tip. The results obtained may be valuable for enhancing our understanding
of the superlubricity of graphite.Comment: 16 pages, 11 figure
Quantum Monte Carlo Calculations for Carbon Nanotubes
We show how lattice Quantum Monte Carlo can be applied to the electronic
properties of carbon nanotubes in the presence of strong electron-electron
correlations. We employ the path-integral formalism and use methods developed
within the lattice QCD community for our numerical work. Our lattice
Hamiltonian is closely related to the hexagonal Hubbard model augmented by a
long-range electron-electron interaction. We apply our method to the
single-quasiparticle spectrum of the (3,3) armchair nanotube configuration, and
consider the effects of strong electron-electron correlations. Our approach is
equally applicable to other nanotubes, as well as to other carbon
nanostructures. We benchmark our Monte Carlo calculations against the two- and
four-site Hubbard models, where a direct numerical solution is feasible.Comment: 54 pages, 16 figures, published in Physical Review
Generation of initial molecular dynamics configurations in arbitrary geometries and in parallel
A computational pre-processing tool for generating initial configurations of molecules for molecular dynamics simulations in geometries described by a mesh of unstructured arbitrary polyhedra is described. The mesh is divided into separate zones and each can be filled with a single crystal lattice of atoms. Each zone is filled by creating an expanding cube of crystal unit cells, initiated from an anchor point for the lattice. Each unit cell places the appropriate atoms for the user-specified crystal structure and orientation. The cube expands until the entire zone is filled with the lattice; zones with concave and disconnected volumes may be filled. When the mesh is spatially decomposed into portions for distributed parallel processing, each portion may be filled independently, meaning that the entire molecular system never needs to fit onto a single processor, allowing very large systems to be created. The computational time required to fill a zone with molecules scales linearly with the number of cells in the zone for a fixed number of molecules, and better than linearly with the number of molecules for a fixed number of mesh cells. Our tool, molConfig, has been implemented in the open source C++ code OpenFOAM
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