7,625 research outputs found
Microsecond long atomistic simulation of supercooled water
Supercooled water is a metastable phase of liquid water below the melting
temperature. An interesting discussion recently developed on the relationship
between crystallization rate and the time scales of equilibration within the
liquid phase. Calculations using a coarse grained monatomic model of water, the
mW model, suggested that equilibration of the liquid below the temperature of
homogeneous nucleation K is slower than ice nucleation. Here, a
3 s long molecular dynamics simulation of the TIP4P-Ew water model is
presented to investigate the relaxation properties of an atomistic model in the
supercooled region below . Evidence is provided that the liquid phase of
the TIP4P-Ew model is at equilibrium in the supercooled regime before ice
nucleation.Comment: 2 pages, 2 figure
Modeling friction: From nanoscale to mesoscale
The physics of sliding friction is gaining impulse from nanoscale and
mesoscale experiments, simulations, and theoretical modeling. This Colloquium
reviews some recent developments in modeling and in atomistic simulation of
friction, covering open-ended directions, unconventional nanofrictional
systems, and unsolved problems.Comment: 26 pages, 14 figures, Rev. Mod. Phys. Colloquiu
Coupling of Length Scales and Atomistic Simulation of MEMS Resonators
We present simulations of the dynamic and temperature dependent behavior of
Micro-Electro-Mechanical Systems (MEMS) by utilizing recently developed
parallel codes which enable a coupling of length scales. The novel techniques
used in this simulation accurately model the behavior of the mechanical
components of MEMS down to the atomic scale. We study the vibrational behavior
of one class of MEMS devices: micron-scale resonators made of silicon and
quartz. The algorithmic and computational avenue applied here represents a
significant departure from the usual finite element approach based on continuum
elastic theory. The approach is to use an atomistic simulation in regions of
significantly anharmonic forces and large surface area to volume ratios or
where internal friction due to defects is anticipated. Peripheral regions of
MEMS which are well-described by continuum elastic theory are simulated using
finite elements for efficiency. Thus, in central regions of the device, the
motion of millions of individual atoms is simulated, while the relatively large
peripheral regions are modeled with finite elements. The two techniques run
concurrently and mesh seamlessly, passing information back and forth. This
coupling of length scales gives a natural domain decomposition, so that the
code runs on multiprocessor workstations and supercomputers. We present novel
simulations of the vibrational behavior of micron-scale silicon and quartz
oscillators. Our results are contrasted with the predictions of continuum
elastic theory as a function of size, and the failure of the continuum
techniques is clear in the limit of small sizes. We also extract the Q value
for the resonators and study the corresponding dissipative processes.Comment: 10 pages, 10 figures, to be published in the proceedings of DTM '99;
LaTeX with spie.sty, bibtex with spiebib.bst and psfi
Nanoscale domains in ionic liquids: A statistical mechanics definition for molecular dynamics studies
One of the many open questions concerning Ionic Liquids (ILs) is the
existence of nanoscale supramolecular domains which characterize the bulk. The
hypothesis of their existence does not meet a general consensus since their
definition seems to be based on ad hoc arbitrary criteria rather than on
general and solid first principles of physics. In this work, we propose a
suitable definition of supramolecular domains based on first principles of
statistical mechanics. Such principles can be realized through the application
of a recently developed computational tool which employs adaptive molecular
resolution. The method can identify the smallest region of a liquid for which
the atomistic details are strictly required, while the exterior plays the role
of a generic structureless thermodynamic reservoir. We consider four different
imidazolium-based ILs and show that indeed one can quantitatively represent the
liquid as a collection of atomistically self-contained nanodroplets embedded in
a generic thermodynamic bath. Such nanodroplets express a characteristic length
scale for heterogeneity in ILs.Comment: 9 page
Efficient 3D `atomistic' simulation technique for studying of random dopant induced threshold voltage lowering and fluctuations in decanano MOSFETs
A 3D `atomistic' simulation technique to study random dopant induced threshold voltage lowering and fluctuations in sub 0.1 μm MOSFETs is presented. It allows statistical analysis of random impurity effects down to the individual impurity level. Efficient algorithms based on a single solution of Poisson's equation, followed by the solution of a simplified current continuity equation are used in the simulations
Three-body Hydrogen Bond Defects Contribute Significantly to the Dielectric Properties of the Liquid Water-Vapor Interface
In this Letter, we present a simple model of aqueous interfacial molecular
structure and we use this model to isolate the effects of hydrogen bonding on
the dielectric properties of the liquid water-vapor interface. By comparing
this model to the results of atomistic simulation we show that the anisotropic
distribution of molecular orientations at the interface can be understood by
considering the behavior of a single water molecule interacting with the
average interfacial density field via an empirical hydrogen bonding potential.
We illustrate that the depth dependence of this orientational anisotropy is
determined by the geometric constraints of hydrogen bonding and we show that
the primary features of simulated orientational distributions can be reproduced
by assuming an idealized, perfectly tetrahedral hydrogen bonding geometry. We
also demonstrate that non-ideal hydrogen bond geometries are required to
produce interfacial variations in the average orientational polarization and
polarizability. We find that these interfacial properties contain significant
contributions from a specific type of geometrically distorted three-body
hydrogen bond defect that is preferentially stabilized at the interface. Our
findings thus reveal that the dielectric properties of the liquid water-vapor
interface are determined by collective molecular interactions that are unique
to the interfacial environment.Comment: 5 pages, 4 figure, S
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