Phase changes in 38 atom Lennard-Jones clusters. II: A parallel tempering study of equilibrium and dynamic properties in the molecular dynamics and microcanonical

We study the 38-atom Lennard-Jones cluster with parallel tempering Monte Carlo methods in the microcanonical and molecular dynamics ensembles. A new Monte Carlo algorithm is presented that samples rigorously the molecular dynamics ensemble for a system at constant total energy, linear and angular momenta. By combining the parallel tempering technique with molecular dynamics methods, we develop a hybrid method to overcome quasi-ergodicity and to extract both equilibrium and dynamical properties from Monte Carlo and molecular dynamics simulations. Several thermodynamic, structural and dynamical properties are investigated for LJ$_{38}$, including the caloric curve, the diffusion constant and the largest Lyapunov exponent. The importance of insuring ergodicity in molecular dynamics simulations is illustrated by comparing the results of ergodic simulations with earlier molecular dynamics simulations.Comment: Journal of Chemical Physics, accepte

Structure and mass transport characteristics of the surface of gadolinium-doped ceria nanocrystals: Molecular dynamics study

Surface of CGO nanocrystals in a vacuum was investigated by MD method in a wide temperature range with five different potentials. Temperature dependences of distances between cations in near-surface layers in the direction along the surface and to the nanocrystal center were studied in detail. Surface area ratio S(100)/S(111) for equilibrium shape of truncated octahedron has maximum for nanocrystals of 6000-12,000 ions. Surface ion displacement was decomposed on the path along the surface and into the bulk of nanocrystal. The diffusion of surface ions in the entire temperature range has predominantly two-dimensional character, regardless of potentials and the dopant concentration X. Anion diffusion coefficients on the surface and in the bulk were compared, they coincide at T < 1200 K for X ≥ 0.05. For a system with X = 0.01 anion migration energies of 0.82 eV and 0.77 eV, and anti-Frenkel pair formation energies of 3.01 eV and 4.04 eV on the surface and in the bulk, respectively, were obtained. © 2013 Elsevier B.V

Modeling, simulation and analysis of the reaction field for electrostatic interactions in aqueous solution

How to deal with the long-range electrostatic interactions theoretically and computationally has been well studied due to their importance in biological processes and time consuming summations in computer simulations. The main focus of our research has been on the design and application of a new type of hybrid model that combines both the explicit and implicit solvent models using a reaction field (RF) approach, for accurate and efficient electrostatic calculations. This hybrid model, named as Image Charge Solvation Model (ICSM), replaces an infinite Coulomb summation by two finite sums over direct interactions plus image charges for RF. To characterize the ICSM, the electrostatic torques and forces using different model parameters are compared through various histogram distributions. The contributions of RF are 20% and 2% of the total electrostatic torques and forces, respectively, suggesting that the main effect of RF is to maintain the orientation of water dipoles in the solution. Considering systematic artifacts of the discontinuous dielectric constant at the edge of the cavity, we modified the image charge formula in an optimal way to better account for the continuously changing dielectric profile near the boundary, which provides a computational procedure to determine the most accurate RF possible for a specified water model. The Periodic Boundary Conditions (PBC) in ICSM reduces the size of the productive region and introduces unphysical correlations between ions in ionic solution. With combination of finite boundary conditions, mean field theory for short-range forces and multiple constraint forces applied to water molecules in a buffer layer, bulk water properties are maintained without problems from imaged ions in a much bigger usable region than before. To summarize, the results presented in this work provide a complete characterization, optimization and improvement of the ICSM for electrostatic calculations

Phase-field approach to polycrystalline solidification including heterogeneous and homogeneous nucleation

Advanced phase-field techniques have been applied to address various aspects of polycrystalline solidification including different modes of crystal nucleation. The height of the nucleation barrier has been determined by solving the appropriate Euler-Lagrange equations. The examples shown include the comparison of various models of homogeneous crystal nucleation with atomistic simulations for the single component hard-sphere fluid. Extending previous work for pure systems (Gránásy L, Pusztai T, Saylor D and Warren J A 2007 Phys. Rev. Lett. 98 art no 035703), heterogeneous nucleation in unary and binary systems is described via introducing boundary conditions that realize the desired contact angle. A quaternion representation of crystallographic orientation of the individual particles (outlined in Pusztai T, Bortel G and Gránásy L 2005 Europhys. Lett. 71 131) has been applied for modeling a broad variety of polycrystalline structures including crystal sheaves, spherulites and those built of crystals with dendritic, cubic, rhombododecahedral, truncated octahedral growth morphologies. Finally, we present illustrative results for dendritic polycrystalline solidification obtained using an atomistic phase-field model

Basic Understanding of Condensed Phases of Matter via Packing Models

Packing problems have been a source of fascination for millenia and their study has produced a rich literature that spans numerous disciplines. Investigations of hard-particle packing models have provided basic insights into the structure and bulk properties of condensed phases of matter, including low-temperature states (e.g., molecular and colloidal liquids, crystals and glasses), multiphase heterogeneous media, granular media, and biological systems. The densest packings are of great interest in pure mathematics, including discrete geometry and number theory. This perspective reviews pertinent theoretical and computational literature concerning the equilibrium, metastable and nonequilibrium packings of hard-particle packings in various Euclidean space dimensions. In the case of jammed packings, emphasis will be placed on the "geometric-structure" approach, which provides a powerful and unified means to quantitatively characterize individual packings via jamming categories and "order" maps. It incorporates extremal jammed states, including the densest packings, maximally random jammed states, and lowest-density jammed structures. Packings of identical spheres, spheres with a size distribution, and nonspherical particles are also surveyed. We close this review by identifying challenges and open questions for future research.Comment: 33 pages, 20 figures, Invited "Perspective" submitted to the Journal of Chemical Physics. arXiv admin note: text overlap with arXiv:1008.298

Acoustical properties of double porosity granular materials

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics