2,696 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
Effect of trainee level on surgical time and postoperative complications of anterior cruciate ligament reconstruction
PURPOSE: The objective of this study was to investigate the association between trainee level and surgical time and postoperative complications of anterior cruciate ligament reconstruction (ACLR).
METHODS: A retrospective chart review of patients who underwent ACLR at an academic orthopaedic ambulatory surgery center collected demographic and clinical information, including the number of trainees present and trainee level. Unadjusted and adjusted regression analyses assessed the association between trainee number and level with surgical time (time from skin incision to closure) and postoperative complications.
RESULTS: Of 799 patients in this study operated on by one of five academic sports surgeons, 87% had at least one trainee involved. The average surgical time overall was 93 ± 21 minutes and by trainee level was 99.7 (junior resident), 88.5 (senior residents), 96.6 (fellows), and 95.6 (no trainees). Trainee level was significantly associated with surgical time (P = 0.0008), with increased surgical time in cases involving fellows (0.0011). Fifteen complications (1.9%) were observed within 90 days of surgery. No notable risk factors of postoperative complications were identified.
CONCLUSION: Resident trainee level does not have a notable effect on surgical time or postoperative complications for ACLR at an ambulatory surgery center, although cases involving fellows had longer surgical times. Trainee level was not associated with risk of postoperative complications
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
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.
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
Analytical response time estimation in parallel relational database systems
Techniques for performance estimation in parallel database systems are well established for parameters such as throughput, bottlenecks and resource utilisation. However, response time estimation is a complex activity which is difficult to predict and has attracted research for a number of years. Simulation is one option for predicting response time but this is a costly process. Analytical modelling is a less expensive option but requires approximations and assumptions about the queueing networks built up in real parallel database machines which are often questionable and few of the papers on analytical approaches are backed by results from validation against real machines. This paper describes a new analytical approach for response time estimation that is based on a detailed study of different approaches and assumptions. The approach has been validated against two commercial parallel DBMSs running on actual parallel machines and is shown to produce acceptable accuracy
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
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
Molecular dynamics study of melting of a bcc metal-vanadium II : thermodynamic melting
We present molecular dynamics simulations of the thermodynamic melting
transition of a bcc metal, vanadium using the Finnis-Sinclair potential. We
studied the structural, transport and energetic properties of slabs made of 27
atomic layers with a free surface. We investigated premelting phenomena at the
low-index surfaces of vanadium; V(111), V(001), and V(011), finding that as the
temperature increases, the V(111) surface disorders first, then the V(100)
surface, while the V(110) surface remains stable up to the melting temperature.
Also, as the temperature increases, the disorder spreads from the surface layer
into the bulk, establishing a thin quasiliquid film in the surface region. We
conclude that the hierarchy of premelting phenomena is inversely proportional
to the surface atomic density, being most pronounced for the V(111) surface
which has the lowest surface density
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