Design and analysis of sandwiched fullerene-graphene composites using molecular dynamics simulations

Computational design of a novel carbon based hybrid material that is composed of fullerene units covalently sandwiched between parallel graphene sheets is presented. In this regard, atomistic models for the proposed novel material structure are generated via a systematic approach by employing different fullerene types (i.e. C₁₈₀, C₃₂₀, C₅₄₀ and C₇₂₀) as sandwich cores. Then, thermodynamic stability of the atomistic structures is checked by monitoring free energy profiles and junctional bond configurations which are obtained through classical molecular dynamics (MD) simulations. Thermodynamic feasibility of all atomistic specimens with different fullerene types is suggested by the energy profiles, because total configuration energies for all systems are minimized and remained stable over a long period of time. Furthermore, mechanical behavior of the nano-sandwiched material system is investigated by performing compression tests via MD simulations and basic deformation mechanisms underlying the compressive response are determined. By detailed examination, it is shown that proposed nano-sandwiched material can be identified as quasi-foam material due to comparable energy absorbing characteristics. Furthermore, regarding the effect of fullerene size on the compressive response, it is found that for a given stress level, specimens with larger fullerenes exhibit higher energy absorbing capacity.     

Mechanical properties of thermoelectric Mg2Si using molecular dynamics simulations

In this paper, the mechanical properties of thermoelectric Mg₂Si are investigated using molecular dynamics (MD) method, with Mg₂Si in the forms of bulk, nanofilm, and nanowire, respectively. The effects of the Mg vacancy on the mechanical properties of Mg₂Si in these three forms are studied in details. First of all, the equilibrium state of Mg₂Si is simulated after choosing the proper potential function, boundary conditions, and the speed algorithm. Nondimensionalization is also implemented during the simulations. This part of simulation aims to verify the correctness of the crystal model established via the use of the molecular dynamics analysis. Next, the models of Mg₂Si in the forms of bulk, nanofilm, and nanowire are established with different Mg vacancy proportions, and then the mechanical properties of each model are studied via the uniaxial tensile test. Finally, the stress–strain curve and subsequently the ultimate tensile strength are obtained for each model. Simulation results indicate that the ultimate tensile strength of Mg₂Si in each model is decreased with the increase of the Mg vacancy proportion. Moreover, through the comparison of the ultimate tensile strengths of Mg₂Si bulk, nanofilm, and nanowire, it is found that low-dimensionalization significantly reduces the ultimate tensile strength of thermoelectric Mg₂Si. Results obtained in this paper can provide valuable guidance to the future applications of thermoelectric devices.     

Tight-binding molecular dynamics for materials simulations

Summary Tight-binding molecular dynamics has recently emerged as a useful method for atomistic simulation of the structural, dynamical and electronic properties of realistic materials. The method incorporates quantum-mechanical calculations into molecular dynamics through an empirical tight-binding Hamiltonian and bridges the gap between ab initio molecular dynamics and simulations using empirical classical potentials. In this paper, we review the accuracy, efficiency, and predictive power of the method and discuss some opportunities and challenges for future development.     

An investigation into the melting of silicon nanoclusters using molecular dynamics simulations

Using the Stillinger-Weber?(SW) potential model, we have performed molecular dynamics?(MD) simulations to investigate the melting of silicon nanoclusters comprising a maximum of 9041 atoms. This study investigates the size, surface energy and root mean square displacement?(RMSD) characteristics of the silicon nanoclusters as they undergo a heating process. The numerical results reveal that an intermediate nanocrystal regime exists for clusters with more than 357?atoms. Within this regime, a linear relationship exists between the cluster size and its melting temperature. It is found that melting of the silicon nanoclusters commences at the surface and that T(m,N) = T(m,Bulk)-αN(-1/3). Therefore, the extrapolated melting temperature of the bulk with a surface decreases from T(m,Bulk) = 1821?K to a value of T(m,357) = 1380?K at the lower limit of the intermediate nanocrystal regime.     

Computational limits of classical molecular dynamics simulations

The system sizes and time scales accessible by classical molecular dynamics techniques on current-generation parallel supercomputers are briefly discussed. The implications for simulation of glasses and glass-forming materials now and in the near future are highlighted.     

Million-atom molecular dynamics simulations of magnetic iron

The problem of large-scale molecular dynamics simulations of iron has recently attracted attention in connection with the need to understand the microscopic picture of radiation damage in ferritic steels. In this paper we review the development of a new interatomic potential for magnetic iron, and describe the first large-scale atomistic simulations performed using the new method. We investigate the structure and thermally activated mobility of self-interstitial atom clusters and show that the spatial distribution of magnetic moments around a cluster is well correlated with the distribution of hydrostatic pressure, highlighting the significant part played by magneto-elasticity in the treatment of radiation damage. We show that self-interstitial atom clusters exhibit a transition from relatively immobile configurations containing 〈1 1 0〉-like groups of atoms to 〈1 1 1〉-like configurations occurring at a critical cluster size N_c ∼ 5 atoms. We discuss implications of this finding for the treatment of cascade damage effects, and the possibility of observing new low-temperature resistivity recovery stages in neutron-irradiated α-iron.     

Evaluation of a grid based molecular dynamics approach for polypeptide simulations

Molecular dynamics is very important for biomedical research because it makes possible simulation of the behavior of a biological macromolecule in silico. However, molecular dynamics is computationally rather expensive: the simulation of some nanoseconds of dynamics for a large macromolecule such as a protein takes very long time, due to the high number of operations that are needed for solving the Newton's equations in the case of a system of thousands of atoms. In order to obtain biologically significant data, it is desirable to use high-performance computation resources to perform these simulations. Recently, a distributed computing approach based on replacing a single long simulation with many independent short trajectories has been introduced, which in many cases provides valuable results. This study concerns the development of an infrastructure to run molecular dynamics simulations on a grid platform in a distributed way. The implemented software allows the parallel submission of different simulations that are singularly short but together bring important biological information. Moreover, each simulation is divided into a chain of jobs to avoid data loss in case of system failure and to contain the dimension of each data transfer from the grid. The results confirm that the distributed approach on grid computing is particularly suitable for molecular dynamics simulations thanks to the elevated scalability.     

Large-scale molecular dynamics simulations of fracture and deformation

Summary We have discussed the prospects of applying massively parallel molecular dynamics simulation to investigate brittle versus ductile fracture behaviors and dislocation intersection. This idea is illustrated by simulating dislocation emission from a three-dimensional crack. Unprecedentedly, the dislocation loops emitted from the crack fronts have been observed. It is found that dislocation-emission modes, jogging or blunting, are very sensitive to boundary conditions and interatomic potentials. These 3D phenomena can be effectively visualized and analyzed by a new technique, namely, plotting only those atoms within the certain ranges of local potential energies.     

Large-scale molecular dynamics simulations of hyperthermal cluster impact

Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.     

Coarse-grained molecular dynamics simulations of ionic polymer networks

The stress-strain behavior of cross-linked polymeric networks was investigated using molecular dynamics simulations with a coarse-grained representation of the repeating units. The network structure was formed by dynamically cross-linking the reactants placed between two rigid layers comprised of particles of the same type. We studied two types of networks which differ only by one containing ionic pairs that amount to 7% of the total number of bonds present. The stress-strain curves were obtained after imposing deformation in tensile and shear modes to the networks and measuring their stress response. Under both forms of deformations there was improvement in the level of stress that the material could bear. Moreover, the time dependent behavior of the improvement in mechanical properties signified a self-healing mechanism.     

Material property prediction of thermoset polymers by molecular dynamics simulations

Molecular dynamics simulations are conducted to predict thermal and mechanical properties of a family of thermoset polymers. We focus on the effect of cross-linkers on density, glass transition temperature, elastic constants, and strength. The polymers are composed of the epoxy resin DGEBA (EPON825) and a series of cross-linkers with different number of active sites and rigidity 33DDS, 44DDS, APB133, TREN, and TAPA. Our simulations quantify effects of cross-linkers on thermal and mechanical properties.     

Million atom molecular dynamics simulations of materials on parallel computers

Recent advances in computing tecnology — parallel computer architectures, portable software and development of robust O(N) algorithms — have revolutionized the field of computer simulation. Using the space-time multiresolution molecular dynamics algorithms it is possible to carry out multimillion atom simulations of materials in different ranges of density, temperature and uniaxial strain.     

Fracture properties of metals and alloys from molecular dynamics simulations

Molecular dynamics simulations of fracture have been performed on the metals Al and Nb, and the intermetallic alloys RuAl, Nb₃Al and NiAl. The forces and energies were modelled with embedded atom method potentials. The increasing external stress was applied using displacements of the outer boundaries of the array, calculated by anisotropic elasticity theory, until the pre-existing cracks propagated or dislocation nucleation occurred. The resulting critical stress intensity factor was calculated at various orientations and temperatures, and the results compared with theory. Observations of slip systems are reported, as well as values for surface energies and “unstable stacking” energies.     

A molecular dynamics approach in simulations of fluid-solid particles flow

In the paper a discrete system of particles carried by fluid is considered in a planar motion. The volumetric density of particles is assumed to be small enough such that they can be treated within the framework of a molecular dynamics model. The fluid is then considered as a carrier of particles. The Landau-Lifshitz concept of turbulence is used to describe the fluctuating part of fluid velocity. This approach is applied to simulate different regimes (laminar and turbulent) and various states of particle motion (moving bed, heterogeneous flow, and homogeneous flow) using only two parameters, which have to be determined experimentally. These two parameters, found for a particular pipe and for a particular velocity from a simple experiment, then can be used for other pipe diameters and different velocities. The computer simulations performed for the flow of particles in pipes at different flow velocities and different pipe diameters agree favorably with experimental observations of the type of flow and critical velocities identifying transitions from one type to another. Received: 8 January 1999     

Application of molecular dynamics simulations for structural studies of carbon nanotubes

Molecular dynamics studies based on the Brenner-Tersoff second-generation reactive empirical bond order potential and the Lennard-Jones carbon-carbon potential for intra- and inter-layer interactions have been performed for carbon nanotubes. These potentials reproduce reasonably the carbon-carbon distances and inter-layer spacing. The structure factors and the reduced radial distribution functions computed from the cartesian coordinates, resulting from energy minimisation and molecular dynamics simulations at 2 K and 300 K have been obtained for two models of two- and five-wall carbon nanotubes containing defects in the form of five and seven membered carbon rings. The results of computations have been compared with experimental data obtained from neutron and X-ray diffraction. The energy relaxation and the molecular dynamics simulations at 2 K and 300 K with appropriate values of the Debye-Waller factor lead practically to the same results which are in a good agreement with the experimental data indicating that molecular dynamics reproduce all structure features of the investigated carbon nanotubes together with thermal oscillations. Possible applications of this approach for other carbon nanotubes and related materials have been also discussed.     

Growth of Ag nanometre-sized particles in solution: molecular dynamics simulations

This work focuses on the growth of nanometre-sized Ag clusters in solution. Molecular dynamics simulations have been employed to gain the necessary detail on the dynamics of solute species and to study the mechanistic features of the processes governing the association of solute atoms in aggregates. Supersaturated liquid solutions of Ag in tetrachloromethane have been considered. A systematic variation of the concentration of Ag atoms in solution permitted us to show the different mechanistic scenarios responsible for the growth processes of solid Ag clusters. It is shown that such processes are limited by the thermal diffusion of solute in the solution bulk at relatively low supersaturation degrees, whereas the growth is limited by interfacial effects at relatively high supersaturation degrees.     

Prediction of thermodynamic properties for fluid nitrogen with molecular dynamics simulations

Molecular dynamics simulation results in the isochoric isothermal ensemble are reported for a two-center Lennard Jones model of fluid nitrogen characterized by the fixed molecular elongationL = 1 = 0.3292, New values of and were determined by fitting the vapor pressure and the saturated liquid density of the model to experimental data at 94,67 K. The required vapor liquid equilibrium data of the model were taken from a study using the NpT + test particle method. The resulting values are k = 36.32013 K (36.673 K) and = 0.32973 nm (0.33073 nm), with values in parentheses being those obtained previously from a Weeks Chandler Andersen-type perturbation theory. Then pressures and internal energies were calculated by molecular simulations for 110 state points in the temperature range from 72 to 330 K and for densities up to 35 mol · L¹. Comparison of the predictions based on the new parameters with the empirical equation of state of Jacobsen et al. shows good to excellent agreement except in the near-critical region. Moreover. for almost all state points the new parameters yield an improvement over old ones from perturbation theory.     

Structural properties of amorphous hydrogenated silicon usingab initio molecular dynamics simulations

We have studied structural properties of amorphous hydrogenated silicon usingab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.