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Félix Mouhat, Sandro Sorella, Rodolphe Vuilleumier, Antonino Marco Saitta, Michele Casula
We introduce a novel approach for a fully quantum description of coupled electron-ion systems from first principles. It combines the variational quantum Monte Carlo solution of the electronic part with the path integral formalism for the quantum nuclear dynamics. On the one hand, the path integral molecular dynamics includes nuclear quantum effects by adding a set of fictitious classical particles (beads) aimed at reproducing nuclear quantum fluctuations via a harmonic kinetic term. On the other hand, variational quantum Monte Carlo can provide Born-Oppenheimer potential energy surfaces with a precision comparable to the most advanced post Hartree-Fock approaches, and with a favorable scaling with the system size...
April 25, 2017: Journal of Chemical Theory and Computation
G Fier, D Hansmann, R C Buceta
In this work we introduce a stochastic model to describe directional changes in the movement of swimming bacteria. We use the probability density function (PDF) of turn angles, measured on tumbling wild-type E. coli, to build a Langevin equation for the deflection of the bacterial body swimming in isotropic media. We have solved this equation analytically by means of the Green function method and shown that three parameters are sufficient to describe the movement: the characteristic time, the steady-state solution and the control parameter...
April 21, 2017: Soft Matter
Yoji Kawamura
We formulate a theory for the collective phase reduction of globally coupled noisy dynamical elements exhibiting macroscopic rhythms. We first transform the Langevin-type equation that represents a group of globally coupled noisy dynamical elements into the corresponding nonlinear Fokker-Planck equation and then develop the phase reduction method for limit-cycle solutions to the nonlinear Fokker-Planck equation. The theory enables us to describe the collective dynamics of a group of globally coupled noisy dynamical elements by a single degree of freedom called the collective phase...
March 2017: Physical Review. E
Serena di Santo, Pablo Villegas, Raffaella Burioni, Miguel A Muñoz
We revisit the problem of deriving the mean-field values of avalanche exponents in systems with absorbing states. These are well known to coincide with those of unbiased branching processes. Here we show that for at least four different universality classes (directed percolation, dynamical percolation, the voter model or compact directed percolation class, and the Manna class of stochastic sandpiles) this common result can be obtained by mapping the corresponding Langevin equations describing each of them into a random walker confined to the origin by a logarithmic potential...
March 2017: Physical Review. E
Andrey A Kuznetsov, Alexander F Pshenichnikov
Langevin dynamics simulation is used to study the suspension of interacting magnetic nanoparticles (dipolar spheres) in a zero applied magnetic field and in the presence of a gravitational (centrifugal) field. A particular emphasis is placed on the equilibrium vertical distribution of particles in the infinite horizontal slab. An increase in the dipolar coupling constant λ (the ratio of dipole-dipole interaction energy to thermal energy) from zero to seven units causes an increase in the particle segregation coefficient by several orders of magnitude...
March 2017: Physical Review. E
Debarshee Bagchi
We study the thermal transport properties of the one-dimensional Fermi-Pasta-Ulam model (β type) with long-range interactions. The strength of the long-range interaction decreases with the (shortest) distance between the lattice sites as distance^{-δ}, where δ≥0. Two Langevin heat baths at unequal temperatures are connected to the ends of the one-dimensional lattice via short-range harmonic interactions that drive the system away from thermal equilibrium. In the nonequilibrium steady state the heat current, thermal conductivity, and temperature profiles are computed by solving the equations of motion numerically...
March 2017: Physical Review. E
Jia-Ming Zhang, Jing-Dong Bao
We study a frequency-dependent damping model of hyperdiffusion within the generalized Langevin equation. The model allows for the colored noise defined by its spectral density, assumed to be proportional to ω^{δ-1} at low frequencies with 0<δ<1 (sub-Ohmic damping) or 1<δ<2 (super-Ohmic damping), where the frequency-dependent damping is deduced from the noise by means of the fluctuation-dissipation theorem. It is shown that for super-Ohmic damping and certain parameters, the diffusive process of the particle in a titled periodic potential undergos sequentially four time regimes: thermalization, hyperdiffusion, collapse, and asymptotical restoration...
March 2017: Physical Review. E
Jacopo Grilli, Matteo Osella, Andrew S Kennard, Marco Cosentino Lagomarsino
A recent burst of dynamic single-cell data makes it possible to characterize the stochastic dynamics of cell division control in bacteria. Different models were used to propose specific mechanisms, but the links between them are poorly explored. The lack of comparative studies makes it difficult to appreciate how well any particular mechanism is supported by the data. Here, we describe a simple and generic framework in which two common formalisms can be used interchangeably: (i) a continuous-time division process described by a hazard function and (ii) a discrete-time equation describing cell size across generations (where the unit of time is a cell cycle)...
March 2017: Physical Review. E
S P Sreenilayam, D M Agra-Kooijman, V P Panov, V Swaminathan, J K Vij, Yu P Panarin, A Kocot, A Panov, D Rodriguez-Lojo, P J Stevenson, Michael R Fisch, Satyendra Kumar
A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA^{*}-SmC^{*} phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA^{*}-SmC^{*} transition, and low values of the reduction factor (∼0.40) suggest that the SmA^{*} phase in this material is of the de Vries type...
March 2017: Physical Review. E
Tomasz Srokowski
The random walk process in a nonhomogeneous medium, characterized by a Lévy stable distribution of jump length, is discussed. The width depends on a position: either before the jump or after that. In the latter case, the density slope is affected by the variable width and the variance may be finite; then all kinds of the anomalous diffusion are predicted. In the former case, only the time characteristics are sensitive to the variable width. The corresponding Langevin equation with different interpretations of the multiplicative noise is discussed...
March 2017: Physical Review. E
T Sakaue, J-C Walter, E Carlon, C Vanderzande
We develop a theoretical description of the critical zipping dynamics of a self-folding polymer. We use tension propagation theory and the formalism of the generalized Langevin equation applied to a polymer that contains two complementary parts which can bind to each other. At the critical temperature, the (un)zipping is unbiased and the two strands open and close as a zipper. The number of broken base pairs n(t) displays a subdiffusive motion characterized by a variance growing as 〈Δn(2)(t)〉 ∼ t(α) with α < 1 at long times...
April 11, 2017: Soft Matter
Michelle C Swift, Marilyn Langevin, Alexander M Clark
A challenge that speech-language pathologists (SLPs) face is the translation of research into clinical practice. While randomised controlled trials (RCTs) are often touted as the "gold standard" of efficacy research, much valuable information is lost through the process; RCTs by nature are designed to wash out individual client factors and contexts that might influence the outcome in order to present the "true" impact of the intervention. However, in the area of behavioural interventions, the interaction of client factors and contexts with the treatment agent can substantially influence the outcome...
April 10, 2017: International Journal of Speech-language Pathology
Zachary E Dell, Kenneth S Schweizer
We develop a segment-scale, force-based theory for the breakdown of the unentangled Rouse model and subsequent emergence of isotropic mesoscopic localization and entropic elasticity in chain polymer liquids in the absence of ergodicity-restoring anisotropic reptation or activated hopping motion. The theory is formulated in terms of a conformational N-dynamic-order-parameter generalized Langevin equation approach. It is implemented using a universal field-theoretic Gaussian thread model of polymer structure and closed at the level of the chain dynamic second moment matrix...
April 7, 2017: Journal of Chemical Physics
Sergei Izvekov
We consider the generalized Langevin equations of motion describing exactly the particle-based coarse-grained dynamics in the classical microscopic ensemble that were derived recently within the Mori-Zwanzig formalism based on new projection operators [S. Izvekov, J. Chem. Phys. 138(13), 134106 (2013)]. The fundamental difference between the new family of projection operators and the standard Zwanzig projection operator used in the past to derive the coarse-grained equations of motion is that the new operators average out the explicit irrelevant trajectories leading to the possibility of solving the projected dynamics exactly...
March 28, 2017: Journal of Chemical Physics
José Rafael Bordin, Leandro Batirolla Krott
We explore the self-assembled morphologies of Janus nanoparticles under cylindrical confinement. Langevin Dynamics simulations are employed to study the behavior of two types of dimers inside cylinders with distinct radius. The first type of nanoparticle was modeled using one monomer that interacts by a standard Lennard Jones potential and another monomer that is modeled using a purely repulsive two length scale shoulder potential. The second type is composed by a Lennard Jones monomer and a repulsive monomer which interacts by the purely repulsive Weeks-Chandler-Andersen potential, which have only one length scale...
April 4, 2017: Journal of Physical Chemistry. B
A Asghari Nejad, H R Askari, H R Baghshahi
We theoretically investigate optical bistability for a hybrid optomechanical system, where two cavities (an optomechanical cavity and a traditional one) are coupled together. A Kerr medium is inserted in the optomechanical cavity and there is an ultra-cold atomic gas inside the other. Dynamics of the system is described by Heisenberg-Langevin equations of motion in the steady-state regime. Bistability is observed in intracavity intensity for the optomechanical cavity, and the response of this phenomenon to optical parameters of the system is investigated...
April 1, 2017: Applied Optics
Helene M Langevin, Rosa N Schnyer
No abstract text is available yet for this article.
April 2017: Journal of Alternative and Complementary Medicine: Research on Paradigm, Practice, and Policy
Jose Manuel Martí, Daniel Martínez-Martínez, Teresa Rubio, César Gracia, Manuel Peña, Amparo Latorre, Andrés Moya, Carlos P Garay
The animal microbiota (including the human microbiota) plays an important role in keeping the physiological status of the host healthy. Research seeks greater insight into whether changes in the composition and function of the microbiota are associated with disease. We analyzed published 16S rRNA and shotgun metagenomic sequencing (SMS) data pertaining to the gut microbiotas of 99 subjects monitored over time. Temporal fluctuations in the microbial composition revealed significant differences due to factors such as dietary changes, antibiotic intake, age, and disease...
March 2017: MSystems
M Hamieh, K D Dorkenoo, G Taupier, Y Henry, D Halley
We investigate the second harmonic generation (SHG) signal in strained Cr2O3 clusters. We show that the SHG signal generated by nanometric Cr2O3 clusters embedded in MgO varies under an applied electric field, at room temperature. The variation of the intensity follows a Langevin law as a function of the electric field, which is consistent with a super-paraelectric clusters assembly. This reveals the presence of a weak spontaneous electric dipole in Cr2O3 when in the shape of highly strained epitaxial clusters, whereas this material does not posses any permanent electric dipole in the bulk phase...
May 24, 2017: Journal of Physics. Condensed Matter: An Institute of Physics Journal
Spencer Langevin, Jonathan S Chang, Stanley Chang
BACKGROUND/PURPOSE: To describe a case of serous retinopathy and associated photoreceptor atrophy after intravenous cisplatin therapy. METHODS: Evaluation was performed using electroretinogram, optical coherence tomography, fundus autofluorescence, and funduscopic examinations to assess the extent of retinal disease, toxicity, and eventual atrophy. RESULTS: A 56 year-old man with metastatic small cell carcinoma with vision changes after initiation of cisplatin therapy...
March 22, 2017: Retinal Cases & Brief Reports
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