Empirical Potential Simulations Research Articles

Empirical potential simulations of interstitial dislocation loops in uranium dioxide

Stoichiometric circular shaped interstitial dislocation loop energies are calculated in stoichiometric UO2 by empirical potential simulation. The Burgers vector directions studied are 〈110〉 and 〈111〉. The main structural properties of each type of interstitial dislocation loop are determined, including stacking fault energy. Defect energies are compared and a maximum size for stable 〈111〉 dislocation loops before transition to 〈110〉 dislocation loops is given. A model of dislocation loop energy based on elasticity theory is then fitted on the basis of these simulation results.

First-Principles Predictions of Structure–Function Relationships of Graphene-Supported Platinum Nanoclusters

Platinum-based materials play an important role as electrocatalysts in energy conversion technologies. Graphene-supported Pt nanoclusters were recently found to be promising electrocatalysts for fuel-cell applications due to their enhanced activity and tolerance to CO poisoning as well as their long-term stability toward sintering. However, structure–function relationships that underpin the improved performance of these catalysts are still not well understood. Here, we employ a combination of empirical potential simulations and density functional theory (DFT) calculations to investigate structure–function relationships of small PtN (N = 2–80) clusters on model carbon (graphene) supports. A bond-order empirical potential is employed within a genetic algorithm to go beyond local optimizations in obtaining minimum-energy structures of PtN clusters on pristine as well as defective graphene supports. Point defects in graphene strongly anchor Pt clusters and also appreciably affect the morphologies of small clu...

Nonlinear anisotropic deformation behavior of a graphene monolayer under uniaxial tension.

Based on first principle calculations, a graphene monolayer shows a complicated deformation behavior under uniaxial tension. The maximum stress of graphene is reached when the bond stretching ratio is far less than its breaking value, which means that graphene shows the typical "ductile-like" behavior but not the conventionally considered "brittle-like" behavior. Although the graphene monolayer shows isotropic behavior in strength, it is strongly anisotropic in deformation (i.e., the ultimate strain is highly different along the different directions). Under uniaxial tension along the zigzag/armchair direction, the overall deformation is only supported by the C-C bonds in one orientation, whereas the C-C bonds in the other orientation and the C-C-C bond angle have almost no contribution, which cannot be correctly predicted by the empirical potential simulations. The complicated bond deformation means that the conventional constitutive model (σ = Eε + Dε(2)) cannot accurately describe the tensile behavior of the graphene monolayer. According to the bond deformation under uniaxial tension, graphene can be simplified as a spring-network including both nonlinear springs (resisting both the tensile and compressive load) and a very strong compressive angle-spring (resisting the decrease of the C-C-C bond angle).

First‐principles and empirical potential simulation study of intrinsic and carbon‐related defects in silicon

AbstractResults of atomistic simulations aimed at understanding precipitation of the highly attractive wide band gap semiconductor material silicon carbide in silicon are presented. The study involves a systematic investigation of intrinsic and carbon‐related defects as well as defect combinations and defect migration by both, quantummechanical first‐principles as well as empirical potential methods. Comparing formation and activation energies, ground‐state structures of defects and defect combinations as well as energetically favorable agglomeration of defects are predicted. Moreover, accurate ab initio calculations unveil limitations of the analytical method based on a Tersoff‐like bond order potential. A work‐around is proposed in order to subsequently apply the highly efficient technique on large structures not accessible by first‐principles methods. The outcome of both types of simulation provides a basic microscopic understanding of defect formation and structural evolution particularly at non‐equilibrium conditions strongly deviated from the ground state as commonly found in SiC growth processes. A possible precipitation mechanism, which conforms well to experimental findings and clarifies contradictory views present in the literature is outlined (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative study of the first-principles and empirical potential simulation of vacancies in silicon

利用第一性原理及Stillinger-Weber（SW）,EDIP和Tersoff经验势函数对比研究了硅中单空位(V1)、双空位(V2)和六边形空位环(V6)的结构特性及形成能.讨论了经验势函数描述空位时的优点和缺点.结果发现，第一性原理方法可以精确描述空位的原子结构及能量特性，而短程有效的经验势函数无法描述空位所固有的量子效应，如Jahn-Teller变形等.另外，由于经验势函数自身的缺陷，EDIP和T3无法应用于空位结构特性的计算.虽然

A theoretical study of intrinsic point defects and defect clusters in magnesium aluminatespinel

Point and small cluster defects in magnesium aluminate spinel have been studiedfrom a first principles viewpoint. Typical point defects that occur during collisioncascade simulations are cation anti-site defects, which have a small formationenergy and are very stable, O and Mg split interstitials and vacancies. IsolatedAl interstitials were found to be energetically unfavourable but could occur aspart of a split Mg–Al pair or as a three atom–three vacancy Al ‘ring’ defect,previously observed in collision cascades using empirical potentials. The structure andenergetics of the defects were investigated using density functional theory (DFT)and the results compared to simulations using empirical fixed charge potentials.Each point defect was studied in a variety of supercell sizes in order to ensureconvergence. It was found that empirical potential simulations significantly overestimateformation energies, but that the type and relative stability of the defects are wellpredicted by the empirical potentials both for point defects and small defect clusters.

Ab initiostudy of point defects in magnesium oxide

Energetics of a variety of point defects in MgO have been considered from an ab initio perspective using density functional theory. The considered defects are isolated Schottky and Frenkel defects and interstitial pairs, along with a number of Schottky defects and di-interstitials. Comparisons were made between the density functional theory results and results obtained from empirical potential simulations and these generally showed good agreement. Both methodologies predicted the first nearest neighbor Schottky defects to be the most energetically favorable of the considered Schottky defects and that the first, second, and fifth nearest neighbor di-interstitials were of similar energy and were favored over the other di-interstitial configurations. Relaxed structures of the defects were analyzed, which showed that empirical potential simulations were accurately predicting the displacements of atoms surrounding di-interstitials, but were overestimating O atom displacement for Schottky defects. Transition barriers were computed for the defects using the nudged elastic band method. Vacancies and Schottky defects were found to have relatively high energy barriers, the majority of which were over $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, in agreement with conclusions reached using empirical potentials. The lowest barriers for di-interstitial transitions were found to be for migration into a first nearest neighbor configuration. Charges were calculated using a Bader analysis and this found negligible charge transfer during the defect transitions and only small changes in the charges on atoms surrounding defects, indicating why fixed charge models work as well as they do.

Atomic and electronic structure of crystalline–amorphous carbon interfaces

Interfaces of diamond with amorphous carbon (a-C) are investigated using tight-binding molecular dynamics simulations. Such interfaces are found to be stable, with a-C atoms covalently bonded to the diamond surfaces. The atomic and electronic structure of the a-C region is consistent with previous results on pure a-C and does not depend critically on the diamond face exposed. However, surface properties influence the relative stability of interfaces with high density a-C. In this case, the interfacial region is small and very dense homogeneous a-C grows on diamond. At lower densities, carbon atoms nucleate on diamond surfaces and create an intermediate region between diamond and a-C. The shape of diamond crystals embedded in a-C is predicted using the Wulff construction with appropriately defined energies for surfaces with overlayer material. These predictions are verified by empirical potential simulations of nanodiamond inclusions in a-C matrix. The electronic density of states and the dielectric function, calculated for our samples containing a-C/diamond interfaces, show that optoelectronic properties of these composite materials are dominated by a-C.

Tight-Binding Atomistic Simulations of Hydrocarbon Sputtering by Hyperthermal Ions in Tokamak Divertors

Recent empirical potential simulations [5] explain from an atomistic point of view the mechanisms of the erosion of carbon-based fusion reactor walls by low-energy hydrogen ions coming from the plasma. However, it is not quite clear yet to what extent the quantum-mechanical effects are important for an adequate description of the erosion. Making use of tight-binding molecular dynamics, which accounts for the quantum-mechanical nature of impinging particle-surface interaction, we re-examine the low-energy erosion mechanisms by simulating the ion-assisted bond breaking in simple model systems like carbon dimers and by modeling sputtering from realistic hydrogenated carbon slabs. Our simulations confirm the empirical potential results on the low energy bond-breaking mechanism, and help one to differentiate between the different mechanisms of the erosion.

F Atom Adsorption on the Fluorinated Si(001) Surface

First-principles pseudopotential total-energy calculations have been performed to investigate the adsorption of a F atom on the F-terminated Si(001)-2×1 surface. We have determined several stable sites for the adsorbed F atom where energy differences and energy barriers between these adsorption sites are relatively small. It is shown that the incoming F atom can be adsorbed on the fully fluorinated Si(001) surface and break the substrate Si–Si bonds, which is contrary to the results of previous empirical potential simulation.

Multiscale Simulations of Brittle Fracture and the Quantum-Mechanical Nature of Bonding in Silicon

AbstractWe simulate the microscopic details of brittle fracture in silicon by dynamically coupling empirical-potential molecular dynamics of a strained sample to a quantum-mechanical description of interatomic bonding at the crack tip. Our simulations show brittle fracture at loads comparable to experiment, in contrast with empirical potential simulations that show only ductile crack propagation at much higher loading. While the ductility of the empirical potentials can be attributed to their short range, it is unclear whether the increased range of the tight-binding description is sufficient to explain its brittle behavior. Using the multiscale method we show that at a temperature of 1100 K, but not at 900 K, a dislocation is sometimes nucleated when the crack tip impinges on a vacancy. While this result is too limited in length and time scales to directly correspond to experimental observations, it is suggestive of the experimentally observed brittle to ductile transition.

HOCl Adsorption on Ice Surfaces

HOCl has been implicated in the destruction of stratospheric ozone via a reaction catalyzed on ice surfaces in polar stratospheric clouds. Adsorption of HOCl on ice is a prototype of a simple ice surface reaction, and it has been the subject of several laboratory studies, making this a valuable case for testing theoretical approaches to ice surface reactions. The present paper describes a first-principles theoretical study of HOCl adsorption on ice, applying density functional theory (DFT) to cluster models of the (0001) surface of ice Ih. In the most stable binding configurations, HOCl acts as a proton donor in a hydrogen bond. The strength of HOCl binding on the surface depends not only on the hydrogen bonding interaction but also on electrostatic and many-body interactions with neighboring water molecules. The results of these DFT calculations are compared to post-Hartree−Fock calculations of the H2O−HOCl complex and to empirical potential simulations of HOCl on ice.