452 research outputs found
Amorphous silica modeled with truncated and screened Coulomb interactions: A molecular dynamics simulation study
We show that finite-range alternatives to the standard long-range BKS pair
potential for silica might be used in molecular dynamics simulations. We study
two such models that can be efficiently simulated since no Ewald summation is
required. We first consider the Wolf method, where the Coulomb interactions are
truncated at a cutoff distance r_c such that the requirement of charge
neutrality holds. Various static and dynamic quantities are computed and
compared to results from simulations using Ewald summations. We find very good
agreement for r_c ~ 10 Angstroms. For lower values of r_c, the long--range
structure is affected which is accompanied by a slight acceleration of dynamic
properties. In a second approach, the Coulomb interaction is replaced by an
effective Yukawa interaction with two new parameters determined by a force
fitting procedure. The same trend as for the Wolf method is seen. However,
slightly larger cutoffs have to be used in order to obtain the same accuracy
with respect to static and dynamic quantities as for the Wolf method.Comment: 10 pages; 11 fig
Between two moments
In this short note, we draw attention to a relation between two Horn
polytopes which is proved in [Chenciner-Jim\'enez P\'erez] as the result on the
one side of a deep combinatorial result in [Fomin,Fulton, Li,Poon], on the
other side of a simple computation involving complex structures. This suggested
an inequality between Littlewood-Richardson coefficients which we prove using
the symmetric characterization of these coefficients given in
[Carr\'e,Leclerc].Comment: 9 pages, 3 figure
The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results
The high-pressure dynamics of a computer-modeled silica melt is studied in
the framework of the mode-coupling theory of the glass transition (MCT) using
static-structure input from molecular-dynamics (MD) computer simulation. The
theory reproduces the experimentally known viscosity minimum (diffusivity
maximum) as a function of density or pressure and explains it in terms of a
corresponding minimum in its critical temperature. This minimum arises from a
gradual change in the equilibrium static structure which shifts from being
dominated by tetrahedral ordering to showing the cageing known from
high-density liquids. The theory is in qualitative agreement with computer
simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in
Journal of Physic
New fitting scheme to obtain effective potential from Car-Parrinello molecular dynamics simulations: Application to silica
A fitting scheme is proposed to obtain effective potentials from
Car-Parrinello molecular dynamics (CPMD) simulations. It is used to
parameterize a new pair potential for silica. MD simulations with this new
potential are done to determine structural and dynamic properties and to
compare these properties to those obtained from CPMD and a MD simulation using
the so-called BKS potential. The new potential reproduces accurately the liquid
structure generated by the CPMD trajectories, the experimental activation
energies for the self-diffusion constants and the experimental density of
amorphous silica. Also lattice parameters and elastic constants of alpha-quartz
are well-reproduced, showing the transferability of the new potential.Comment: 6 pages, 5 figure
Asymptotics of Selberg-like integrals: The unitary case and Newton's interpolation formula
We investigate the asymptotic behavior of the Selberg-like integral ,
as for different scalings of the parameters and with .
Integrals of this type arise in the random matrix theory of electronic
scattering in chaotic cavities supporting channels in the two attached
leads. Making use of Newton's interpolation formula, we show that an asymptotic
limit exists and we compute it explicitly
Attosecond chirp-encoded dynamics of light nuclei Attosecond chirp-encoded dynamics of light nuclei
International audienceWe study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D 2 and H 2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H 2 and D 2 calculated theoretically is consistent with our experimental results
Molecular dynamics computer simulation of amorphous silica under high pressure
The structural and dynamic properties of silica melts under high pressure are
studied using molecular dynamics (MD) computer simulation. The interactions
between the ions are modeled by a pairwise-additive potential, the so-called
CHIK potential, that has been recently proposed by Carre et al. The
experimental equation of state is well-reproduced by the CHIK model. With
increasing pressure (density), the structure changes from a tetrahedral network
to a network containing a high number of five- and six-fold Si-O coordination.
In the partial static structure factors, this change of the structure with
increasing density is reflected by a shift of the first sharp diffraction peak
towards higher wavenumbers q, eventually merging with the main peak at
densities around 4.2 g/cm^3. The self-diffusion constants as a function of
pressure show the experimentally-known maximum, occurring around a pressure of
about 20 GPa.Comment: 12 pages, 7 figures, to be published in J. Phys.: Condens. Matte
The influence of tennis court surfaces on player perceptions and biomechanical response.
This study aimed to examine player perceptions and biomechanical responses to tennis surfaces and to evaluate the influence of prior clay court experience. Two groups with different clay experiences (experience group, n = 5 and low-experience group, n = 5) performed a 180° turning movement. Three-dimensional ankle and knee movements (50 Hz), plantar pressure of the turning step (100 Hz) and perception data (visual analogue scale questionnaire) were collected for two tennis courts (acrylic and clay). Greater initial knee flexion (acrylic 20. 8 ± 11.2° and clay 32.5 ± 9.4°) and a more upright position were reported on the clay compared to the acrylic court (P < 0.05). This suggests adaptations to increase player stability on clay. Greater hallux pressures and lower midfoot pressures were observed on the clay court, allowing for sliding whilst providing grip at the forefoot. Players with prior clay court experience exhibited later peak knee flexion compared to those with low experience. All participants perceived the differences in surface properties between courts and thus responded appropriately to these differences. The level of previous clay court experience did not influence players' perceptions of the surfaces; however, those with greater clay court experience may reduce injury risk as a result of reduced loading through later peak knee flexion
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