Search results for "Statistical physics"
showing 10 items of 1402 documents
Critical Dynamics in a Binary Fluid: Simulations and Finite-Size Scaling
2006
We report comprehensive simulations of the critical dynamics of a symmetric binary Lennard-Jones mixture near its consolute point. The self-diffusion coefficient exhibits no detectable anomaly. The data for the shear viscosity and the mutual-diffusion coefficient are fully consistent with the asymptotic power laws and amplitudes predicted by renormalization-group and mode-coupling theories {\it provided} finite-size effects and the background contribution to the relevant Onsager coefficient are suitably accounted for. This resolves a controversy raised by recent molecular simulations.
Single trajectory characterization via machine learning
2020
[EN] In order to study transport in complex environments, it is extremely important to determine the physical mechanism underlying diffusion and precisely characterize its nature and parameters. Often, this task is strongly impacted by data consisting of trajectories with short length (either due to brief recordings or previous trajectory segmentation) and limited localization precision. In this paper, we propose a machine learning method based on a random forest architecture, which is able to associate single trajectories to the underlying diffusion mechanism with high accuracy. In addition, the algorithm is able to determine the anomalous exponent with a small error, thus inherently provi…
Ground states of ultrasoft particles with attractions: a genetic algorithm approach
2009
International audience; We analyze in detail the ground-state structure of model systems of athermal star polymers with an additional, tunable attraction that may result from dispersion or depletion forces. To perform a free, unbiased search in the space spanned by the crystal parameters, we employ genetic algorithms, which are enhanced with respect to previous versions in their ability to find stable structures that occupy very narrow basins of attraction in the energy landscape. Application of this method brings about a very large variety of ground states for star polymers with attractions, in particular for the case of intermediate functionalities and strong, short-range attractive force…
Inverse task for evaluation of particle size distribution of polydisperse magnetic fluids
2010
AbstractThe method of inverse task was used to analyze three different physical phenomena. The particle size distributions were reconstructed from the magnetization curve, dynamic light scattering and magnetic birefringence relaxation data. The results thus obtained for one real magnetic fluid sample are different; they characterize the physical nature of the phenomena. All three methods may be used to determine intrinsic sample properties.
Transition state ensemble optimization for reactions of arbitrary complexity.
2015
In the present work, we use Variational Transition State Theory (VTST) to develop a practical method for transition state ensemble optimization by looking for an optimal hyperplanar dividing surface in a space of meaningful trial collective variables. These might be interatomic distances, angles, electrostatic potentials, etc. Restrained molecular dynamics simulations are used to obtain on-the-fly estimates of ensemble averages that guide the variations of the hyperplane maximizing the transmission coefficient. A central result of our work is an expression that quantitatively estimates the importance of the coordinates used for the localization of the transition state ensemble. Starting fro…
Monte Carlo Simulation of Polymeric Materials — Still a Challenge?
1992
Monte Carlo simulation of polymeric materials is difficult, since they exhibit nontrivial structure over many different length scales, from the bond length (∼1A) to the radius of the random coil (∼102A) and still larger collective length scales, and similarly, motions occur on very different time scales. Hence it is a nontrivial problem to devise suitable coarse-grained models which capture the essential physics and are accessible to simulation.
Theoretical Description of Primary Nanoferroics. Comparison of the Theory with Experiment
2013
This Chapter is devoted primarily to the theoretical description of the physical properties of nanoferroics. The theoretical approach that has been successful in describing the size- and shape-dependent effects observed experimentally in nanoferroics is Landau – Ginzburg – Devonshire phenomenological theory, operating on nanoferroics symmetry and order parameters. Our analysis of this theory applicability shows that it can be safely applied down to the sample sizes of few nanometers. The main peculiarity of theoretical description of nanoferroics is that the boundary conditions and terms containing gradients of order parameters cannot be omitted and play the vital role in the description of…
Universal Dynamic Fragmentation inDDimensions
2004
A generic model is introduced for brittle fragmentation in $D$ dimensions, and this model is shown to lead to a fragment-size distribution with two distinct components. In the small fragment-size limit a scale-invariant size distribution results from a crack branching-merging process. At larger sizes the distribution becomes exponential as a result of a Poisson process, which introduces a large-scale cutoff. Numerical simulations are used to demonstrate the validity of the distribution for $D=2$. Data from laboratory-scale experiments and large-scale quarry blastings of granitic gneiss confirm its validity for $D=3$. In the experiments the nonzero grain size of rock causes deviation from th…
Elastic constants from microscopic strain fluctuations
1999
Fluctuations of the instantaneous local Lagrangian strain $\epsilon_{ij}(\bf{r},t)$, measured with respect to a static ``reference'' lattice, are used to obtain accurate estimates of the elastic constants of model solids from atomistic computer simulations. The measured strains are systematically coarse- grained by averaging them within subsystems (of size $L_b$) of a system (of total size $L$) in the canonical ensemble. Using a simple finite size scaling theory we predict the behaviour of the fluctuations $$ as a function of $L_b/L$ and extract elastic constants of the system {\em in the thermodynamic limit} at nonzero temperature. Our method is simple to implement, efficient and general e…
STRUCTURAL INSTABILITY IN FERROELECTRICS: SUPERIMPOSING HAMILTONIAN AND STOCHASTIC DYNAMICS
2008
ABSTRACT Structural instability of ferroelectrics distinguished by appearance of coexisting phases and spatial inhomogeneity is at variance with the predictions of statistics in the canonical ensemble. A more refined description includes ergodicity breaking which become apparent at critical temperature when the system resides in metastable state and its development lead to one of possible minimum energy states. In this study the domain growth and switching is reproduced within the framework of Fokker-Planck approach. The mathematical technique is developed for empiric Landau Hamiltonians and improved for application to first principles effective Hamiltonians with supercells and elementary l…