30,488 research outputs found
Force correlations in molecular and stochastic dynamics
A molecular gas system in three dimensions is numerically studied by the
energy conserving molecular dynamics (MD). The autocorrelation functions for
the velocity and the force are computed and the friction coefficient is
estimated. From the comparison with the stochastic dynamics (SD) of a Brownian
particle, it is shown that the force correlation function in MD is different
from the delta-function force correlation in SD in short time scale. However,
as the measurement time scale is increased further, the ensemble equivalence
between the microcanonical MD and the canonical SD is restored. We also discuss
the practical implication of the result.Comment: 9 pages, 4 figures and Computer Physics Communcations (in press
Master equation for the probability distribution functions of forces in soft particle packings
Employing molecular dynamics simulations of jammed soft particles, we study
microscopic responses of force-chain networks to quasi-static isotropic
(de)compressions. We show that not only contacts but also interparticle gaps
between the nearest neighbors must be considered for the stochastic evolution
of the probability distribution functions (PDFs) of forces, where the mutual
exchange of contacts and interparticle gaps, i.e. opening and closing contacts,
are also crucial to the incremental system behaviors. By numerically
determining the transition rates for all changes of contacts and gaps, we
formulate a Master equation for the PDFs of forces, where the insight one gets
from the transition rates is striking: The mean change of forces reflects
non-affine system response, while their fluctuations obey uncorrelated Gaussian
statistics. In contrast, interparticle gaps are reacting mostly affine in
average, but imply multi-scale correlations according to a wider stable
distribution function.Comment: 5 pages, 4 figures, submitted to Soft Matte
Dynamic Implicit-Solvent Coarse-Grained Models of Lipid Bilayer Membranes : Fluctuating Hydrodynamics Thermostat
Many coarse-grained models have been developed for equilibrium studies of
lipid bilayer membranes. To achieve in simulations access to length-scales and
time-scales difficult to attain in fully atomistic molecular dynamics, these
coarse-grained models provide a reduced description of the molecular degrees of
freedom and often remove entirely representation of the solvent degrees of
freedom. In such implicit-solvent models the solvent contributions are treated
through effective interaction terms within an effective potential for the free
energy. For investigations of kinetics, Langevin dynamics is often used.
However, for many dynamical processes within bilayers this approach is
insufficient since it neglects important correlations and dynamical
contributions that are missing as a result of the momentum transfer that would
have occurred through the solvent. To address this issue, we introduce a new
thermostat based on fluctuating hydrodynamics for dynamic simulations of
implicit-solvent coarse-grained models. Our approach couples the coarse-grained
degrees of freedom to a stochastic continuum field that accounts for both the
solvent hydrodynamics and thermal fluctuations. We show our approach captures
important correlations in the dynamics of lipid bilayers that are missing in
simulations performed using conventional Langevin dynamics. For both planar
bilayer sheets and bilayer vesicles, we investigate the diffusivity of lipids,
spatial correlations, and lipid flow within the bilayer. The presented
fluctuating hydrodynamics approaches provide a promising way to extend
implicit-solvent coarse-grained lipid models for use in studies of dynamical
processes within bilayers
A Stochastic Immersed Boundary Method for Fluid-Structure Dynamics at Microscopic Length Scales
In this work it is shown how the immersed boundary method of (Peskin2002) for
modeling flexible structures immersed in a fluid can be extended to include
thermal fluctuations. A stochastic numerical method is proposed which deals
with stiffness in the system of equations by handling systematically the
statistical contributions of the fastest dynamics of the fluid and immersed
structures over long time steps. An important feature of the numerical method
is that time steps can be taken in which the degrees of freedom of the fluid
are completely underresolved, partially resolved, or fully resolved while
retaining a good level of accuracy. Error estimates in each of these regimes
are given for the method. A number of theoretical and numerical checks are
furthermore performed to assess its physical fidelity. For a conservative
force, the method is found to simulate particles with the correct Boltzmann
equilibrium statistics. It is shown in three dimensions that the diffusion of
immersed particles simulated with the method has the correct scaling in the
physical parameters. The method is also shown to reproduce a well-known
hydrodynamic effect of a Brownian particle in which the velocity
autocorrelation function exhibits an algebraic tau^(-3/2) decay for long times.
A few preliminary results are presented for more complex systems which
demonstrate some potential application areas of the method.Comment: 52 pages, 11 figures, published in journal of computational physic
The Stochastic Dynamics of an Array of Atomic Force Microscopes in a Viscous Fluid
We consider the stochastic dynamics of an array of two closely spaced atomic
force microscope cantilevers in a viscous fluid for use as a possible
biomolecule sensor. The cantilevers are not driven externally, as is common in
applications of atomic force microscopy, and we explore the stochastic
cantilever dynamics due to the constant buffeting of fluid particles by
Brownian motion. The stochastic dynamics of two adjacent cantilevers are
correlated due to long range effects of the viscous fluid. Using a recently
proposed thermodynamic approach the hydrodynamic correlations are quantified
for precise experimental conditions through deterministic numerical
simulations. Results are presented for an array of two readily available atomic
force microscope cantilevers. It is shown that the force on a cantilever due to
the fluid correlations with an adjacent cantilever is more than 3 times smaller
than the Brownian force on an individual cantilever. Our results indicate that
measurements of the correlations in the displacement of an array of atomic
force microscopes can detect piconewton forces with microsecond time
resolution.Comment: 7 page article with 11 images submitted to the International Journal
of Nonlinear Mechanic
Probing complex RNA structures by mechanical force
RNA secondary structures of increasing complexity are probed combining single
molecule stretching experiments and stochastic unfolding/refolding simulations.
We find that force-induced unfolding pathways cannot usually be interpretated
by solely invoking successive openings of native helices. Indeed, typical
force-extension responses of complex RNA molecules are largely shaped by
stretching-induced, long-lived intermediates including non-native helices. This
is first shown for a set of generic structural motifs found in larger RNA
structures, and then for Escherichia coli's 1540-base long 16S ribosomal RNA,
which exhibits a surprisingly well-structured and reproducible unfolding
pathway under mechanical stretching. Using out-of-equilibrium stochastic
simulations, we demonstrate that these experimental results reflect the slow
relaxation of RNA structural rearrangements. Hence, micromanipulations of
single RNA molecules probe both their native structures and long-lived
intermediates, so-called "kinetic traps", thereby capturing -at the single
molecular level- the hallmark of RNA folding/unfolding dynamics.Comment: 9 pages, 9 figure
Comparison of dynamical multifragmentation models
Multifragmentation scenarios, as predicted by antisymmetrized molecular
dynamics (AMD) or momentum-dependent stochastic mean-field (BGBD) calculations
are compared. While in the BGBD case fragment emission is clearly linked to the
spinodal decomposition mechanism, i.e. to mean-field instabilities, in AMD
many-body correlations have a stronger impact on the fragmentation dynamics and
clusters start to appear at earlier times. As a consequence, fragments are
formed on shorter time scales in AMD, on about equal footing of light particle
pre-equilibrium emission. Conversely, in BGBD pre-equilibrium and fragment
emissions happen on different time scales and are related to different
mechanisms
Equilibrium free energies from fast-switching trajectories with large time steps
Jarzynski's identity for the free energy difference between two equilibrium
states can be viewed as a special case of a more general procedure based on
phase space mappings. Solving a system's equation of motion by approximate
means generates a mapping that is perfectly valid for this purpose, regardless
of how closely the solution mimics true time evolution. We exploit this fact,
using crudely dynamical trajectories to compute free energy differences that
are in principle exact. Numerical simulations show that Newton's equation can
be discretized to low order over very large time steps (limited only by the
computer's ability to represent resulting values of dynamical variables)
without sacrificing thermodynamic accuracy. For computing the reversible work
required to move a particle through a dense liquid, these calculations are more
efficient than conventional fast switching simulations by more than an order of
magnitude. We also explore consequences of the phase space mapping perspective
for systems at equilibrium, deriving an exact expression for the statistics of
energy fluctuations in simulated conservative systems
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