2,403 research outputs found
Evaluation of Constant Potential Method in Simulating Electric Double-Layer Capacitors
A major challenge in the molecular simulation of electric double layer
capacitors (EDLCs) is the choice of an appropriate model for the electrode.
Typically, in such simulations the electrode surface is modeled using a uniform
fixed charge on each of the electrode atoms, which ignores the electrode
response to local charge fluctuations induced by charge fluctuations in the
electrolyte. In this work, we evaluate and compare this Fixed Charge Method
(FCM) with the more realistic Constant Potential Method (CPM), [Reed, et al.,
J. Chem. Phys., 126, 084704 (2007)], in which the electrode charges fluctuate
in order to maintain constant electric potential in each electrode. For this
comparison, we utilize a simplified LiClO-acetonitrile/graphite EDLC. At
low potential difference (), the two methods yield
essentially identical results for ion and solvent density profiles; however,
significant differences appear at higher . At ,
the CPM ion density profiles show significant enhancement (over FCM) of
"partially electrode solvated" Li ions very close to the electrode surface.
The ability of the CPM electrode to respond to local charge fluctuations in the
electrolyte is seen to significantly lower the energy (and barrier) for the
approach of Li ions to the electrode surface.Comment: Corrected typo
A molecular dynamics study on the equilibrium magnetization properties and structure of ferrofluids
We investigate in detail the initial susceptibility, magnetization curves,
and microstructure of ferrofluids in various concentration and particle dipole
moment ranges by means of molecular dynamics simulations. We use the Ewald
summation for the long-range dipolar interactions, take explicitly into account
the translational and rotational degrees of freedom, coupled to a Langevin
thermostat. When the dipolar interaction energy is comparable with the thermal
energy, the simulation results on the magnetization properties agree with the
theoretical predictions very well. For stronger dipolar couplings, however, we
find systematic deviations from the theoretical curves. We analyze in detail
the observed microstructure of the fluids under different conditions. The
formation of clusters is found to enhance the magnetization at weak fields and
thus leads to a larger initial susceptibility. The influence of the particle
aggregation is isolated by studying ferro-solids, which consist of magnetic
dipoles frozen in at random locations but which are free to rotate. Due to the
artificial suppression of clusters in ferro-solids the observed susceptibility
is considerably lowered when compared to ferrofluids.Comment: 33 pages including 12 figures, requires RevTex
Model Channel Ion Currents in NaCl - SPC/E Solution with Applied-Field Molecular Dynamics
Using periodic boundary conditions and a constant applied field, we have
simulated current flow through an 8.125 Angstrom internal diameter, rigid,
atomistic channel with polar walls in a rigid membrane using explicit ions and
SPC/E water. Channel and bath currents were computed from ten 10-ns
trajectories for each of 10 different conditions of concentration and applied
voltage. An electric field was applied uniformly throughout the system to all
mobile atoms. On average, the resultant net electric field falls primarily
across the membrane channel, as expected for two conductive baths separated by
a membrane capacitance. The channel is rarely occupied by more than one ion.
Current-voltage relations are concentration-dependent and superlinear at high
concentrations.Comment: Accepted for publication in Biophysical Journa
Compressive auto-indexing in femtosecond nanocrystallography
Ultrafast nanocrystallography has the potential to revolutionize biology by
enabling structural elucidation of proteins for which it is possible to grow
crystals with 10 or fewer unit cells on the side. The success of
nanocrystallography depends on robust orientation-determination procedures that
allow us to average diffraction data from multiple nanocrystals to produce a
three dimensional (3D) diffraction data volume with a high signal-to-noise
ratio. Such a 3D diffraction volume can then be phased using standard
crystallographic techniques. "Indexing" algorithms used in crystallography
enable orientation determination of diffraction data from a single crystal when
a relatively large number of reflections are recorded. Here we show that it is
possible to obtain the exact lattice geometry from a smaller number of
measurements than standard approaches using a basis pursuit solver.Comment: Spence Festschrift on Ultramicroscop
Amorphous silica between confining walls and under shear: a computer simulation study
Molecular dynamics computer simulations are used to investigate a silica melt
confined between walls at equilibrium and in a steady-state Poisseuille flow.
The walls consist of point particles forming a rigid face-centered cubic
lattice and the interaction of the walls with the melt atoms is modelled such
that the wall particles have only a weak bonding to those in the melt, i.e.
much weaker than the covalent bonding of a Si-O unit. We observe a pronounced
layering of the melt near the walls. This layering, as seen in the total
density profile, has a very irregular character which can be attributed to a
preferred orientational ordering of SiO4 tetrahedra near the wall. On
intermediate length scales, the structure of the melt at the walls can be well
distinguished from that of the bulk by means of the ring size distribution.
Whereas essentially no structural changes occur in the bulk under the influence
of the shear fields considered, strong structural rearrangements in the ring
size distribution are present at the walls as far as there is a slip motion.
For the sheared system, parabolic velocity profiles are found in the bulk
region as expected from hydrodynamics and the values for the shear viscosity as
extracted from those profiles are in good agreement with those obtained in pure
bulk simulations from the appropriate Green-Kubo formula.Comment: 23 pages of Late
Strongly Charged, Flexible Polyelectrolytes in Poor Solvents -- Molecular Dynamics Simulations
We present a set of molecular dynamics (MD) simulations of strongly charged,
flexible polyelectrolyte chains under poor solvent conditions in a salt free
solution. Structural properties of the chains and of the solutions are
reported. By varying the polymer density and the electrostatic interaction
strength we study the crossover from a dominating electrostatic interaction to
the regime of strong screening, where the hydrophobic interactions dominate.
During the crossover a multitude of structures is observed. In the limit of low
polymer density strongly stretched, necklace like conformations are found. In
the opposite limit of high polymer density which is equivalent to strongly
screened electrostatic interactions, we find that the chains are extremely
collapsed, however we observe no agglomeration or phase separation. The
investigations show that the density of free charges is one of the relevant
parameters which rules the behavior of the system and hence should be used as a
parameter to explain experimental results.Comment: 42 pages, including 22 figures and 2 table
Ewald methods for inverse power-law interactions in tridimensional and quasi-two dimensional systems
In this paper, we derive the Ewald method for inverse power-law interactions
in quasi-two dimensional systems. The derivation is done by using two different
analytical methods. The first uses the Parry's limit, that considers the Ewald
methods for quasi-two dimensional systems as a limit of the Ewald methods for
tridimensional systems, the second uses Poisson-Jacobi identities for lattice
sums. Taking into account the equivalence of both derivations, we obtain a new
analytical Fourier transform intregral involving incomplete gamma function.
Energies of the generalized restrictive primitive model of electrolytes
(-RPM) and of the generalized one component plasma model (-OCP) are
given for the tridimensional, quasi-two dimensional and monolayers systems. Few
numerical results, using Monte-Carlo simulations, for -RPM and -OCP
monolayers systems are reported.Comment: to be published in Journal of Physics A: Mathematical and Theoretical
(19 pages, 2 figures and 3 tables
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