500 research outputs found
Finite field methods for the supercell modeling of charged insulator/electrolyte interfaces
Surfaces of ionic solids interacting with an ionic solution can build up charge by exchange of ions. The surface charge is compensated by a strip of excess charge at the border of the electrolyte forming an electric double layer. These electric double layers are very hard to model using the supercell's methods of computational condensed phase science. The problem arises when the solid is an electric insulator (as most ionic solids are) permitting a finite interior electric field over the width of the slab representing the solid in the supercell. The slab acts as a capacitor. The stored charge is a deficit in the solution failing to compensate fully for the solid surface charge. Here, we show how these problems can be overcome using the finite field methods developed by Stengel, Spaldin, and Vanderbilt [Nat. Phys. 5, 304 (2009)]. We also show how the capacitance of the double layer can be computed once overall electric neutrality of the double layer is restored by application of a finite macroscopic field or alternatively by zero electric displacement . The method is validated for a classical model of a solid-electrolyte interface using the finite-temperature molecular dynamics adaptation of the constant field method presented previously [C. Zhang and M. Sprik, Phys. Rev. B 93, 144201 (2016)]. Because ions in electrolytes can diffuse across supercell boundaries, this application turns out to be a critical illustration of the multivaluedness of polarization in periodic systems.Research fellowship (No. ZH 477/1-1) provided by German Research Foundation (DFG) for CZ is gratefully acknowledged
Computing the dielectric constant of liquid water at constant dielectric displacement
This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.93.144201The static dielectric constant of liquid water is computed using classical
force field based molecular dynamics simulation at fixed electric displacement
D. The method to constrain the electric displacement is the finite temperature
classical variant of the constant-D method developed by Stengel, Spaldin and
Vanderbilt (Nat. Phys. 2009, 5: 304). There is also a modification of this
scheme imposing fixed values of the macroscopic field E. The method is applied
to the popular SPC/E model of liquid water. We compare four different estimates
of the dielectric constant, two obtained from fluctuations of the polarization
at D = 0 and E = 0 and two from the variation of polarization with finite D and
E. It is found that all four estimates agree when properly converged. The
computational effort to achieve convergence varies however, with constant D
calculations being substantially more efficient. We attribute this difference
to the much shorter relaxation time of longitudinal polarization compared to
transverse polarization accelerating constant D calculations.Research fellowship (Grant No. ZH 477/1-1) provided by Deutsche Forschungsgemeinschaft (DFG) for C.Z. is gratefully acknowledged
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Finite field formalism for bulk electrolyte solutions
The manner in which electrolyte solutions respond to electric fields is
crucial to understanding the behavior of these systems both at, and away from,
equilibrium. The present formulation of linear response theory for such systems
is inconsistent with common molecular dynamics (MD) implementations. Using the
finite field formalism, suitably adapted for finite temperature MD, we
investigate the response of bulk aqueous NaCl solutions to both finite Maxwell
() and electric displacement () fields. The constant
Hamiltonian allows us to derive the linear response relation for
the ionic conductivity in a simple manner that is consistent with the forces
used in conventional MD simulations. Simulations of a simple point charge model
of an electrolyte solution at constant yield conductivities at
infinite dilution within 15% of experimental values. The finite field approach
also allows us to measure the solvent's dielectric constant from its
polarization response, which is seen to decrease with increasing ionic
strength. Comparison of the dielectric constant measured from polarization
response versus polarization fluctuations enables direct evaluation of the
dynamic contribution to this dielectric decrement, which we find to be small
but not insignificant. Using the constant formulation, we also
rederive the Stillinger-Lovett conditions, which place strict constraints on
the coupling between solvent and ionic polarization fluctuations.We are grateful for computational support from the UK Materials and Molecular Modelling Hub, which is partially funded by EPSRC (Grant No. EP/P020194), for which access was obtained via the UKCP consortium and funded by EPSRC Grant Ref. No. EP/P022561/1. S.J.C. is supported by a Royal Commission for the Exhibition of 1851 Research Fellowship
Tunneling and delocalization in hydrogen bonded systems: a study in position and momentum space
Novel experimental and computational studies have uncovered the proton
momentum distribution in hydrogen bonded systems. In this work, we utilize
recently developed open path integral Car-Parrinello molecular dynamics
methodology in order to study the momentum distribution in phases of high
pressure ice. Some of these phases exhibit symmetric hydrogen bonds and quantum
tunneling. We find that the symmetric hydrogen bonded phase possesses a
narrowed momentum distribution as compared with a covalently bonded phase, in
agreement with recent experimental findings. The signatures of tunneling that
we observe are a narrowed distribution in the low-to-intermediate momentum
region, with a tail that extends to match the result of the covalently bonded
state. The transition to tunneling behavior shows similarity to features
observed in recent experiments performed on confined water. We corroborate our
ice simulations with a study of a particle in a model one-dimensional double
well potential that mimics some of the effects observed in bulk simulations.
The temperature dependence of the momentum distribution in the one-dimensional
model allows for the differentiation between ground state and mixed state
tunneling effects.Comment: 14 pages, 13 figure
Finite Maxwell field and electric displacement Hamiltonians derived from a current dependent Lagrangian
In the common Ewald summation technique for the evaluation of electrostatic forces, the average electric field E is strictly zero. Finite uniform E can be accounted for by adding it as a new degree of freedom in an extended Lagrangian. Representing the uniform polarization P as the time integral of the internal current and E as the time derivative of a uniform vector field A, we define such an extended Lagrangian coupling A to the total current j_t(internal plus external) and hence derive a Hamiltonian resembling the minimal coupling Hamiltonian of electrodynamics. Next, applying a procedure borrowed from nonrelativistic molecular electrodynamics the j_t · A coupling is transformed to P · D form where D is the electric displacement acting as an electrostatic boundary condition. The resulting Hamiltonian is identical to the constant-D Hamiltonian obtained by Stengel, Spaldin and Vanderbilt (SSV) using thermodynamic arguments. The corresponding SSV constant E Hamiltonian is derived from an alternative extended Lagrangian
Viscoelastic response of sonic band-gap materials
A brief report is presented on the effect of viscoelastic losses in a high
density contrast sonic band-gap material of close-packed rubber spheres in air.
The scattering properties of such a material are computed with an on-shell
multiple scattering method, properties which are compared with the lossless
case. The existence of an appreciable omnidirectional gap in the transmission
spectrum, when losses are present, is also reported.Comment: 5 pages, 4 figures, submitted to PR
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Water adsorption on the P-rich GaP(100) surface: Optical spectroscopy from first principles
The contact of water with semiconductors typically changes its surface
electronic structure by oxidation or corrosion processes. A detailed knowledge
- or even control of - the surface structure is highly desirable, as it impacts
the performance of opto-electronic devices from gas-sensing to energy
conversion applications. It is also a prerequisite for density functional
theory-based modelling of the electronic structure in contact with an
electrolyte. The P-rich GaP(100) surface is extraordinary with respect to its
contact with gas-phase water, as it undergoes a surface reordering, but does
not oxidise. We investigate the underlying changes of the surface in contact
with water by means of theoretically derived reflection anisotropy spectroscopy
(RAS). A comparison of our results with experiment reveals that a water-induced
hydrogen-rich phase on the surface is compatible with the boundary conditions
from experiment, reproducing the optical spectra. We discuss potential reaction
paths that comprise a water-enhanced hydrogen mobility on the surface. Our
results also show that computational RAS - required for the interpretation of
experimental signatures - is feasible for GaP in contact with water double
layers. Here, RAS is sensitive to surface electric fields, which are an
important ingredient of the Helmholtz-layer. This paves the way for future
investigations of RAS at the semiconductor-electrolyte interface
ARRA Material Handling Equipment Composite Data Products: Data through Quarter 2 of 2013
This report includes 47 composite data products (CDPs) produced for American Recovery and Reinvestment Act (ARRA) fuel cell material handling equipment, with data through the second quarter of 2013
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