LAMMPS Code Research Articles

Atomistic modeling of thermo‐mechanical properties of cubic SiC

AbstractSiC is an important multifunctional material with application in electronics and as a structural material. Many investigations of SiC have been done using both classical molecular dynamics and first principles methods. However, they are of limited scope and, in particular, SiC properties at finite temperatures have not been adequately evaluated. The good mechanical, thermal, and chemical properties of SiC such as high stiffness, high hardness, high mechanical strength at high temperature, and high thermal conductivity, make SiC a candidate for various applications in nuclear industries. In this work, we evaluated thermomechanical properties at finite temperatures obtained by LAMMPS code with traditionally used Tersoff potential (TR89 with PRB 41 correction), and the newer GW 2002 (GW02) potential. We compared them with the calculations made using MEAM 1995 (MEAM 95) and with our first principles DFT predictions. It is demonstrated that the thermal expansion and mechanical properties calculated as a function of temperature for classical potentials TR89 and GW02 do not agree well with first principles calculations while better agreement is found for the MEAM95 potential. Classical molecular dynamics calculations made with the use of two earlier potentials under‐predict thermal conductivity by one order of magnitude for the TR89 potential and by more than 30% for the GW02.

Mechanical Properties of Schwarzites - A Fully Atomistic Reactive Molecular Dynamics Investigation

ABSTRACTSchwarzites are crystalline, 3D porous structures with a stable negative curvature formed of sp2-hybridized carbon atoms. These structures present topologies with tunable porous size and shape and unusual mechanical properties. In this work, we have investigated the mechanical behavior under compressive strain and energy absorption of four different Schwarzites. We considered two Schwarzites families, the so-called Gyroid and Primitive and two structures from each family. We carried out reactive molecular dynamics simulations, using the ReaxFF force field as available in the LAMMPS code. Our results also show they exhibit remarkable resilience under mechanical compression. They can be reduced to half of their original size before structural failure (fracture) occurs.

Development of a Charge-Implicit ReaxFF Potential for Hydrocarbon Systems.

Molecular dynamics (MD) simulations continue to make important contributions to understanding chemical and physical processes. Concomitant with the growth of MD simulations is the need to have interaction potentials that both represent the chemistry of the system and are computationally efficient. We propose a modification to the ReaxFF potential for carbon and hydrogen that eliminates the time-consuming charge equilibration, eliminates the acknowledged flaws of the electronegativity equalization method, includes an expanded training set for condensed phases, has a repulsive wall for simulations of energetic particle bombardment, and is compatible with the LAMMPS code. This charge-implicit ReaxFF potential is five times faster than the conventional ReaxFF potential for a simulation of keV particle bombardment with a sample size of over 800 000 atoms.

Energy-free machine learning force field for aluminum

We used the machine learning technique of Li et al. (PRL 114, 2015) for molecular dynamics simulations. Atomic configurations were described by feature matrix based on internal vectors, and linear regression was used as a learning technique. We implemented this approach in the LAMMPS code. The method was applied to crystalline and liquid aluminum and uranium at different temperatures and densities, and showed the highest accuracy among different published potentials. Phonon density of states, entropy and melting temperature of aluminum were calculated using this machine learning potential. The results are in excellent agreement with experimental data and results of full ab initio calculations.

Static investigation of adsorption and hetero‐diffusion of copper, silver, and gold adatoms on the (111) surface

In this work, we have used the static molecular simulations combined with an interatomic potential derived from the embedded‐atom method to study the adsorption and hetero‐diffusion on the (111) surface of Cu, Ag, and Au adatoms by using LAMMPS code. The investigation is performed for six heterogeneous systems such as Ag/Au(111), Ag/Cu(111), Au/Ag(111), Au/Cu(111), Cu/Ag(111), and Cu/Au(111). First, we have investigated the relaxation trends and the bond lengths of the atoms in the systems. The calculation results show that, the top layer spacing between the first and second layers of the Au(111), Ag(111), and Cu(111) substrates is contracted. This contraction is found to be more important in the Au(111) substrate. On the other hand, the strong reduction of the binding length is found in Au/Cu(111) for the different adsorption sites. In addition, the binding, adsorption, and static activation energies for all studied systems were examined. The results indicated that the binding and adsorption energies reached their maximum values in the Au/Cu(111) and Au/Ag(111) systems, respectively. Moreover, the static activation barriers for hopping diffusion on the (111) surfaces are found to be low compared with those found in the (100) and (110) surfaces. Therefore, our calculations showed that the difference in energy between the hcp and fcc sites on the (111) surfaces is very small. Copyright © 2017 John Wiley & Sons, Ltd.

Mechanical behaviour of carbon nanotubes under combined twisting–bending

The main objective of this paper is to investigate the mechanical behaviour (strength and stiffness) of carbon nanotubes (CNTs) under combinations of bending and twisting. In order to achieve this goal, molecular dynamics (MD) simulations of bended and twisted CNTs are performed. The LAMMPS code is used, the AIREBO potential is considered for CC bonds, the temperature is kept at 300K and incremental bending and twisting rotations are imposed to the CNT. Two types of CNTs are analyzed, including zig-zag (8,0) and armchair (5,5) CNTs with similar radius and length. The CNTs are also analyzed for pure bending and pure twisting. The main results are shown in the form of diagrams of energy and moment against imposed rotations. Some relevant conclusions are drawn concerning the influence of loading (bending and twisting) on the stiffness, strength and failure of CNTs: namely, it is concluded that armchair CNTs possess higher strength and fracture toughness under twisting–bending loading than zigzag CNTs; additionally, it is found that both CNTs (armchair and zigzag) still support moderate-to-high bending levels without failure after being extremely twisted and torsionally buckled, even for twisting angles four times those corresponding to torsional buckling; finally, the results prove that CNTs, mostly armchair ones, exhibit very high twisting–bending stiffness and strength and can be used with confidence as torsional spring elements in nanoelectromechanical systems (NEMS).

Atomistic Simulation of Fatigue Crack Growth in α-Fe under High Temperature

The crack growth behaviors loaded in mode I under strain and stress control at different temperatures were presented in α-Fe by atomistic simulations using LAMMPS code. The interatomic bonds of atoms were characterized using the embedded atom method interatomic potential. The simulation models were built with initial edge crack subjecting to cyclic uniaxial constant strain rate and constant stress. A temperature range from 100 K to 1200 K was considered to probe the influence of the temperature on crack growth. The crack growth mechanism and the radial distribution function (RDF) during crack growth were investigated. The results indicated that the crack propagation mechanisms were sensitive to temperature and the boundary conditions. By proposed image adjusting technology the dislocation slip bands can be more clearly displayed on screen. In order to include the effect of temperature on crack growth, a temperature factor defined as a function of temperature in exponential form was introduced to modify the theoretical expressions based on thermal activation theory. Its coefficient and index can be determined by the RDF peak value obtained from atomistic simulations. For cyclic loading the crack growth process was dependent on both temperature and cyclic loading period in terms of simulations.

Mechanical Properties and Fracture Dynamics of Silicene Membranes

ABSTRACTThe advent of graphene created a new era in materials science. Graphene is a two-dimensional planar honeycomb array of carbon atoms in sp2-hybridized states. A natural question is whether other elements of the IV-group of the periodic table (such as silicon and germanium), could also form graphene-like structures. Structurally, the silicon equivalent to graphene is called silicene. Silicene was theoretically predicted in 1994 and recently experimentally realized by different groups. Similarly to graphene, silicene exhibits electronic and mechanical properties that can be exploited to nanoelectronics applications.In this work we have investigated, through fully atomistic molecular dynamics (MD) simulations, the mechanical properties of single-layer silicene under mechanical strain. These simulations were carried out using a reactive force field (ReaxFF), as implemented in the LAMMPS code. We have calculated the elastic properties and the fracture patterns.Our results show that the dynamics of the whole fracturing processes of silicene present some similarities with that of graphene as well as some unique features.

Structure impact on the thermal and electronic properties of bismuth telluride by ab-initio and molecular dynamics calculations

We use molecular dynamics and ab-initio methods to predict the thermal and electronic properties of new materials with high figures of merit. The simulated systems are bulk bismuth tellurides with antisite and vacancy defects. Optimizations of the materials under investigation are performed by the SIESTA code for subsequent calculations of force constants, electronic properties, and Seebeck coefficients. The prediction of the thermal conductivity is made by Non-Equilibrium Molecular Dynamics (NEMD) using the LAMMPS code. The thermal conductivity of bulk bismuth telluride with different stoichiometry and with a number of substitution defects is calculated. We have found that the thermal conductivity can be decreased by 60% by introducing vacancy defects. The calculated thermal conductivities for the different structures are compared with the available experimental and theoretical results.

Tension–twisting dependent kinematics of chiral CNTs

This paper shows that chiral carbon nanotubes (CNTs) under twisting show highly dependent strength and fracture toughness on (i) the level of axial tension and (ii) the direction of twisting. To achieve this conclusion, numerous computational intensive molecular dynamics simulations of chiral (6,3) CNTs were performed using the LAMMPS code and the AIREBO potential for C–C bonds. First, we have studied the influence of the tension level and the direction of twisting on the buckling and failure of the chiral CNTs. We show that the applied torques and angles of twist at the buckling and failure stages strongly depend on the level of tensile loading and on the twist direction. In order to explain such noticeable anisotropic behaviour, we have studied the evolution of two kinematic variables (C–C bond length and hexagonal cell angle) as a function of the twist–tension rate. We conclude that the anisotropic failure of chiral CNTs strongly depends on the type of rupture mechanism, which varies with (i) the level of axial tension and (ii) the direction of twisting. Finally, we summarize the information by means of tension–twisting interaction diagrams, which are very dissimilar for direct and inverse twisting. We conclude that inverse twisting influences very negatively the collapse behaviour of chiral CNTs under tension. On the other hand, for low-to-moderate twist–tension rates, the coupling between tension and direct twisting is highly beneficial for the behaviour of chiral CNTs against collapse. This scenario changes dramatically for moderate-to-high twist–tension rates, as the coupling between tension and direct twisting becomes highly detrimental for the chiral CNT collapse process. These original findings might be relevant for the design of nano-devices made of chiral CNTs, as we show that the fracture toughness of chiral CNTs under tension can be highly increased if low-to-moderate direct twisting is added.

Molecular dynamics simulation of deuterium trapping and bubble formation in tungsten

The interaction between plasma particles and tungsten as plasma facing material is one of the critical issues in successfully using tungsten in Tokamak reactors environment. The deuterium bombardment of monocrystalline tungsten was modeled by molecular dynamics simulation using LAMMPS code and Tersoff type interatomic potential. The deuterium trapping rate, implantation depth, and the stopping time in tungsten at several temperatures ranging from 600 to 2000K bombarded by 5–100eV deuterium atoms were simulated. Deuterium bubble formation at near tungsten surface was also studied. Irradiated monocrystalline tungsten became amorphous state prior to deuterium cluster formation, and gas bubbles were observed in the 600, 900, and 1200K tungsten samples. The formation of gas bubbles were caused by the near surface deuterium super-saturation region and the subsequent plastic deformation induced by the local high gas pressure.