1,168 research outputs found
Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation
Parallelism of tight-binding molecular dynamics simulations is presented by
means of the order-N electronic structure theory with the Wannier states,
recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is
tested for silicon nanocrystals of more than millions atoms with the
transferable tight-binding Hamiltonian. The efficiency of parallelism is
perfect, 98.8 %, and the method is the most suitable to parallel computation.
The elapse time for a system of atoms is 3.0 minutes by a
computer system of 64 processors of SGI Origin 3800. The calculated results are
in good agreement with the results of the exact diagonalization, with an error
of 2 % for the lattice constant and errors less than 10 % for elastic
constants.Comment: 5 pages, 3 figure
Highly efficient and genotype-independent barley gene-editing based on anther culture
Recalcitrance to tissue culture and genetic transformation is the major bottleneck for gene manipulation in crops. In barley, immature embryos of Golden Promise have been mainly used as explants for transformation. However, the genotype-dependent approach limits the genetic modification in commercial varieties. Here, we develop an anther culture-based system that effectively creates transgenic and gene-edited plants from commercial barley varieties. The protocol was tested in four Australian varieties and Golden Promise with different phenology, callus induction and green plant regeneration responses. Agrobacterium-mediated transformation was performed on microspore-derived callus when targeting the HvPDS gene, and T0 albinos with targeted mutations were successfully obtained from commercial varieties. Further editing of three targets was achieved with an average mutation rate of 53% in the five varieties. In the 51 analysed T0 individuals, Cas9 induced about 69% of single-base insertion/deletions and two-base deletions in targeting sites, with variable mutation rates among targets and varieties. Both on-target and off-target activities were detected in T1 progenies. Compared with immature embryo protocols, this genotype-independent platform can deliver a high editing efficiency and more regenerants within a similar time frame. It is promising for functional genomics and application of CRISPR technologies for precise improvement in commercial varieties
An Empirical Charge Transfer Potential with Correct Dissociation Limits
The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has
always embodied charge transfer processes. The mechanism of that behavior is
examined here and recast for use as a new empirical potential energy surface
for large-scale simulations. A two-state model is explored. The main features
of the model are: (1) Explicit decomposition of the total system electron
density is invoked; (2) The charge is defined through the density decomposition
into constituent contributions; (3) The charge transfer behavior is controlled
through the resonance energy matrix elements which cannot be ignored; and (4) A
reference-state approach, similar in spirit to the EVB method, is used to
define the resonance state energy contributions in terms of "knowable"
quantities. With equal validity, the new potential energy can be expressed as a
nonthermal ensemble average with a nonlinear but analytical charge dependence
in the occupation number. Dissociation to neutral species for a gas-phase
process is preserved. A variant of constrained search density functional theory
is advocated as the preferred way to define an energy for a given charge.Comment: Submitted to J. Chem. Phys. 11/12/03. 14 pages, 8 figure
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
Van der Waals loops and the melting transition in two dimensions
Evidence for the existence of van der Waals loops in pressure p versus volume
v plots has for some time supported the belief that melting in two dimensions
is a first order phase transition. We report rather accurate equilibrium p(v)
curves for systems of hard disks obtained from long Monte Carlo simulations.
These curves, obtained in the constant volume ensemble, using periodic boundary
conditions, exhibit well defined van der Waals loops. We illustrate their
existence for finite systems that are known to undergo a continuous transition
in the thermodynamic limit. To this end, we obtain magnetization m versus
applied field curves from Monte Carlo simulations of the 2D Ising model, in the
constant m ensemble, at the critical point. Whether van der Waals loops for
disk systems behave in the thermodynamic limit as they do for the 2D Ising
model at the critical point cannot be ruled out. Thus, the often made claim
that melting in 2D is a first order phase transition, based on the evidence
that van der Waals loops exist, is not sound.Comment: 10 pages, 6 Postscript figures (submitted to Phys.Rev.E). For related
work, see http://pipe.unizar.es/~jf
Absence of a Finite-Temperature Melting Transition in the Classical Two-Dimensional One-Component Plasma
Vortices in thin-film superconductors are often modelled as a system of
particles interacting via a repulsive logarithmic potential. Arguments are
presented to show that the hypothetical (Abrikosov) crystalline state for such
particles is unstable at any finite temperature against proliferation of
screened disclinations. The correlation length of crystalline order is
predicted to grow as as the temperature is reduced to zero, in
excellent agreement with our simulations of this two-dimensional system.Comment: 3 figure
Anharmonic Decay of Vibrational States in Amorphous Silicon
Anharmonic decay rates are calculated for a realistic atomic model of
amorphous silicon. The results show that the vibrational states decay on
picosecond timescales and follow the two-mode density of states, similar to
crystalline silicon, but somewhat faster. Surprisingly little change occurs for
localized states. These results disagree with a recent experiment.Comment: 10 pages, 4 Postscript figure
Simulations of Two-Dimensional Melting on the Surface of a Sphere
We have simulated a system of classical particles confined on the surface of
a sphere interacting with a repulsive potential. The same system
simulated on a plane with periodic boundary conditions has van der Waals loops
in pressure-density plots which are usually interpreted as evidence for a first
order melting transition, but on the sphere such loops are absent.
We also investigated the structure factor and from the width of the first
peak as a function of density we can show that the growth of the correlation
length is consistent with KTHNY theory. This suggests that simulations of two
dimensional melting phenomena are best performed on the surface of a sphere.Comment: 4 eps figure
Measuring kinetic coefficients by molecular dynamics simulation of zone melting
Molecular dynamics simulations are performed to measure the kinetic
coefficient at the solid-liquid interface in pure gold. Results are obtained
for the (111), (100) and (110) orientations. Both Au(100) and Au(110) are in
reasonable agreement with the law proposed for collision-limited growth. For
Au(111), stacking fault domains form, as first reported by Burke, Broughton and
Gilmer [J. Chem. Phys. {\bf 89}, 1030 (1988)]. The consequence on the kinetics
of this interface is dramatic: the measured kinetic coefficient is three times
smaller than that predicted by collision-limited growth. Finally,
crystallization and melting are found to be always asymmetrical but here again
the effect is much more pronounced for the (111) orientation.Comment: 8 pages, 9 figures (for fig. 8 : [email protected]). Accepted for
publication in Phys. Rev.
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