5,646 research outputs found
Bayesian ensemble refinement by replica simulations and reweighting
We describe different Bayesian ensemble refinement methods, examine their
interrelation, and discuss their practical application. With ensemble
refinement, the properties of dynamic and partially disordered (bio)molecular
structures can be characterized by integrating a wide range of experimental
data, including measurements of ensemble-averaged observables. We start from a
Bayesian formulation in which the posterior is a functional that ranks
different configuration space distributions. By maximizing this posterior, we
derive an optimal Bayesian ensemble distribution. For discrete configurations,
this optimal distribution is identical to that obtained by the maximum entropy
"ensemble refinement of SAXS" (EROS) formulation. Bayesian replica ensemble
refinement enhances the sampling of relevant configurations by imposing
restraints on averages of observables in coupled replica molecular dynamics
simulations. We show that the strength of the restraint should scale linearly
with the number of replicas to ensure convergence to the optimal Bayesian
result in the limit of infinitely many replicas. In the "Bayesian inference of
ensembles" (BioEn) method, we combine the replica and EROS approaches to
accelerate the convergence. An adaptive algorithm can be used to sample
directly from the optimal ensemble, without replicas. We discuss the
incorporation of single-molecule measurements and dynamic observables such as
relaxation parameters. The theoretical analysis of different Bayesian ensemble
refinement approaches provides a basis for practical applications and a
starting point for further investigations.Comment: Paper submitted to The Journal of Chemical Physics (15 pages, 4
figures); updated references; expanded discussions of related formalisms,
error treatment, and ensemble refinement with and without replicas; appendi
Coarse Molecular Dynamics of a Peptide Fragment: Free Energy, Kinetics, and Long-Time Dynamics Computations
We present a ``coarse molecular dynamics'' approach and apply it to studying
the kinetics and thermodynamics of a peptide fragment dissolved in water. Short
bursts of appropriately initialized simulations are used to infer the
deterministic and stochastic components of the peptide motion parametrized by
an appropriate set of coarse variables. Techniques from traditional numerical
analysis (Newton-Raphson, coarse projective integration) are thus enabled;
these techniques help analyze important features of the free-energy landscape
(coarse transition states, eigenvalues and eigenvectors, transition rates,
etc.). Reverse integration of (irreversible) expected coarse variables backward
in time can assist escape from free energy minima and trace low-dimensional
free energy surfaces. To illustrate the ``coarse molecular dynamics'' approach,
we combine multiple short (0.5-ps) replica simulations to map the free energy
surface of the ``alanine dipeptide'' in water, and to determine the ~ 1/(1000
ps) rate of interconversion between the two stable configurational basins
corresponding to the alpha-helical and extended minima.Comment: The article has been submitted to "The Journal of Chemical Physics.
Hydrodynamics of Diffusion in Lipid Membrane Simulations
By performing molecular dynamics simulations with up to 132 million
coarse-grained particles in half-micron sized boxes, we show that hydrodynamics
quantitatively explains the finite-size effects on diffusion of lipids,
proteins, and carbon nanotubes in membranes. The resulting Oseen correction
allows us to extract infinite-system diffusion coefficients and membrane
surface viscosities from membrane simulations despite the logarithmic
divergence of apparent diffusivities with increasing box width. The
hydrodynamic theory of diffusion applies also to membranes with asymmetric
leaflets and embedded proteins, and to a complex plasma-membrane mimetic
Scanner
An aerial vehicle rotating in gyroscopic fashion about one of its axes has an optical system which scans an area below the vehicle in determined relation to vehicle rotation. A sensing device is provided to sense the physical condition of the area of scan and optical means are associated to direct the physical intelligence received from the scan area to the sensing means. Means are provided to incrementally move the optical means through a series of steps to effect sequential line scan of the area being viewed keyed to the rotational rate of the vehicle
Pair diffusion, hydrodynamic interactions, and available volume in dense fluids
We calculate the pair diffusion coefficient D(r) as a function of the
distance r between two hard-sphere particles in a dense monodisperse
suspension. The distance-dependent pair diffusion coefficient describes the
hydrodynamic interactions between particles in a fluid that are central to
theories of polymer and colloid dynamics. We determine D(r) from the
propagators (Green's functions) of particle pairs obtained from discontinuous
molecular dynamics simulations. At distances exceeding 3 molecular diameters,
the calculated pair diffusion coefficients are in excellent agreement with
predictions from exact macroscopic hydrodynamic theory for large Brownian
particles suspended in a solvent bath, as well as the Oseen approximation.
However, the asymptotic 1/r distance dependence of D(r) associated with
hydrodynamic effects emerges only after the pair distance dynamics has been
followed for relatively long times, indicating non-negligible memory effects in
the pair diffusion at short times. Deviations of the calculated D(r) from the
hydrodynamic models at short distances r reflect the underlying many-body fluid
structure, and are found to be correlated to differences in the local available
volume. The procedure used here to determine the pair diffusion coefficients
can also be used for single-particle diffusion in confinement with spherical
symmetry.Comment: 6 pages, 5 figure
Resonant Scattering and Ly-alpha Radiation Emergent from Neutral Hydrogen Halos
With a state-of-the-art numerical method for solving the
integral-differential equation of radiative transfer, we investigate the flux
of the Ly photon emergent from an optically thick halo
containing a central light source. Our focus is on the time-dependent effects
of the resonant scattering. We first show that the frequency distribution of
photons in the halo are quickly approaching to a locally thermalized state
around the resonant frequency, even when the mean intensity of the radiation is
highly time-dependent. Since initial conditions are forgotten during the
thermalization, some features of the flux, such as the two peak structure of
its profile, actually are independent of the intrinsic width and time behavior
of the central source, if the emergent photons are mainly from photons in the
thermalized state. In this case, the difference , where
are the frequencies of the two peaks of the flux, cannot be less
than times of Doppler broadening. We then study the radiative transfer in
the case where the light emitted from the central source is a flash. We
calculate the light curves of the flux from the halo. It shows that the flux is
still a flash. The time duration of the flash for the flux, however, is
independent of the original time duration of the light source but depends on
the optical depth of the halo. Therefore, the spatial transfer of resonant
photons is a diffusion process, even though it is not a purely Brownian
diffusion. This property enables an optically thick halo to trap and store
thermalized photons around for a long time after the cease of the
central source emission. The photons trapped in the halo can yield delayed
emission, of which the profile also shows typical two peak structure as that
from locally thermalized photons. Possible applications of these results are
addressed.Comment: 25 pages, 10 figures, accepted for publication in Ap
Vibrational and optical properties of MoS: from monolayer to bulk
Molybdenum disulfide, MoS2, has recently gained considerable attention as a
layered material where neighboring layers are only weakly interacting and can
easily slide against each other. Therefore, mechanical exfoliation allows the
fabrication of single and multi-layers and opens the possibility to generate
atomically thin crystals with outstanding properties. In contrast to graphene,
it has an optical gap of 1.9 eV. This makes it a prominent candidate for
transistor and opto-electronic applications. Single-layer MoS exhibits
remarkably different physical properties compared to bulk MoS due to the
absence of interlayer hybridization. For instance, while the band gap of bulk
and multi-layer MoS is indirect, it becomes direct with decreasing number
of layers. In this review, we analyze from a theoretical point of view the
electronic, optical, and vibrational properties of single-layer, few-layer and
bulk MoS. In particular, we focus on the effects of spin-orbit interaction,
number of layers, and applied tensile strain on the vibrational and optical
properties. We examine the results obtained by different methodologies, mainly
ab initio approaches. We also discuss which approximations are suitable for
MoS and layered materials. The effect of external strain on the band gap of
single-layer MoS and the crossover from indirect to direct band gap is
investigated. We analyze the excitonic effects on the absorption spectra. The
main features, such as the double peak at the absorption threshold and the
high-energy exciton are presented. Furthermore, we report on the phonon
dispersion relations of single-layer, few-layer and bulk MoS. Based on the
latter, we explain the behavior of the Raman-active and
modes as a function of the number of layers
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