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Zigzag hexagonal boron nitride nanoribbon (h-BNNR) model in two-dimensional (2D) case is studied via molecular dynamics simulation. The model contains 10000 atoms interacted via the Tersoff bond order potential. Temperature is increased from 50 K to 7000 K in order to see the evolution of various thermodynamic quantities, structural characteristics, and the occurrence of liquid-like atoms upon heating to a molten state. Lindemann criterion for 2D case is calculated and used to observe the appearance of liquid-like atoms. Atomic mechanism is analyzed on the basis of the occurrence/growth of liquid-like atoms, the formation of clusters, coordination number, and ring statistics. The largest cluster does not contain all liquid-like atoms in the whole range of temperature in this study. After reaching a peak at the melting point of this model, the largest cluster slightly decreases indicating that the single largest cluster of liquid-like atoms tends to form a ring-like 2D liquid zigzag h-BNNR. Armchair h-BNNR is studied to compare with zigzag h-BNNR model. The melting point of armchair h-BNNR (3600 K) is close to the experimental results of hexagonal boron nitride (h-BN) (from 2800 K to 3600 K) whereas the one of zigzag h-BNNR (4522 K) is higher than that of armchair h-BNNR and plane h-BN.

Bond-order potentials (BOPs) provide a local and physically transparent description of the interatomic interaction. Here we describe the efficient implementation of analytic BOPs in the BOPfox program and library. We discuss the integration of the underlying non-magnetic, collinear-magnetic and noncollinear-magnetic tight-binding models that are evaluated by the analytic BOPs. We summarise the flow of an analytic BOP calculation including the determination of self-returning paths for computing the moments, the self-consistency cycle, the estimation of the band-width from the recursion coefficients, and the termination of the BOP expansion. We discuss the implementation of the calculations of forces, stresses and magnetic torques with analytic BOPs. We show the scaling of analytic BOP calculations with the number of atoms and moments, present options for speeding up the calculations and outline different concepts of parallelisation. In the appendix we compile the implemented equations of the analytic BOP methodology and comments on the implementation. This description should be relevant for other implementations and further developments of analytic bond-order potentials.

Magnesium-based materials provide some of the highest capacities for solid-state hydrogen storage. However, efforts to improve their performance rely on a comprehensive understanding of thermodynamic and kinetic limitations at various stages of (de)hydrogenation. Part of the complexity arises from the fact that unlike interstitial metal hydrides that retain the same crystal structures of the underlying metals, MgH2 and other magnesium-based hydrides typically undergo dehydrogenation reactions that are coupled to a structural phase transformation. As a first step towards enabling molecular dynamics studies of thermodynamics, kinetics, and (de)hydrogenation mechanisms of Mg-based solid-state hydrogen storage materials with changing crystal structures, we have developed an analytical bond order potential for Mg-H systems. We demonstrate that our potential accurately reproduces property trends of a variety of elemental and compound configurations with different coordinations, including small clusters and bulk lattices. More importantly, we show that our potential captures the relevant (de)hydrogenation chemical reactions 2H (gas)→H2 (gas) and 2H (gas)+Mg (hcp)→MgH2 (rutile) within molecular dynamics simulations. This verifies that our potential correctly prescribes the lowest Gibbs free energies to the equilibrium H2 and MgH2 phases as compared to other configurations. It also indicates that our molecular dynamics methods can directly reveal atomic processes of (de)hydrogenation of the Mg-H systems.

Nanostructures of transition metal di-chalcogenides (TMDCs) exhibit exotic thermal, chemical and electronic properties, enabling diverse applications from thermoelectrics and catalysis to nanoelectronics. The thermal properties of these nanoscale TMDCs are of particular interest for thermoelectric applications. Thermal transport studies on nanotubes and nanoribbons remain intractable to first principles calculations whereas existing classical molecular models treat the two chalcogen layers in a monolayer with different atom types; this imposes serious limitations in studying multi-layered TMDCs and dynamical phenomena such as nucleation and growth. Here, we overcome these limitations using machine learning (ML) and introduce a bond order potential (BOP) trained against first principles training data to capture the structure, dynamics, and thermal transport properties of a model TMDC such as WSe2. The training is performed using a hierarchical objective genetic algorithm workflow to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet. As a representative case study, we perform molecular dynamics simulations using the ML-BOP model to study the structure and temperature-dependent thermal conductivity of WSe2 tubes and ribbons of different chiralities. We observe slightly higher thermal conductivities along the armchair direction than zigzag for WSe2 monolayers but the opposite effect for nanotubes, especially of smaller diameters. We trace the origin of these differences to the anisotropy in thermal transport and the restricted momentum selection rules for phonon–phonon Umpklapp scattering. The developed ML-BOP model is of broad interest and will facilitate studies on nucleation and growth of low dimensional WSe2 structures as well as their transport properties for thermoelectric and thermal management applications.

An interatomic potential for iron-yttrium is presented that allows to investigate the diffusion of yttrium in iron. The potential provides a complete description of the elastic and thermal properties of yttrium. The iron-yttrium interaction is fitted with focus on yttrium point defects in an iron matrix, as the interaction and diffusion of point defects is the crucial part of the formation of precipitates in ODS steels. The formation energy of substitutional yttrium atoms, the binding energy to vacancies and the migration barriers are very well reproduced. The potential is used to investigate the interaction between substitutional yttrium atoms and edge dislocations. It shows a significant attraction between yttrium atoms and the stress field of the dislocation which leads to yttrium segregation and pinning of dislocation motion. Pipe diffusion of yttrium atoms within the edge dislocation core is investigated by calculating migration barriers. The potential predicts a significant reduction of the activation barriers in the tensile part of the stress field around an edge dislocation. We conclude that yttrium pipe diffusion is relevant for the precipitate formation in ODS steels.

We use the coexisting phases approach to calculate melting phase diagrams of several Fe-C interaction potentials, such as Embedded Atom Method (EAM) potential of Lau et al. [Phys. Rev. Lett. 98 (2007) 215501], EAM potential of Hepburn and Ackland [Phys. Rev. B 78 (2008) 165115], and two flavours of the Analytic Bond Order potential (ABOP) of Henriksson and Nordlund [Phys. Rev. B 79 (2009) 144107]. Melting of both bcc (ferrite) and fcc (austenite) crystals is investigated with C concentrations up to 5 wt%. The results are compared with the experimental data and suggest that the potential of Hepburn and Ackland is the most accurate in reproducing the melting phase diagram of the ferrite, although the austenite cannot be stabilized at any C concentration for this potential. The potential of Lau et al. yields the best qualitative agreement with the real phase diagram in that the ferrite-liquid coexistence at low C concentrations is replaced by the austenite-liquid coexistence at higher C concentrations. However, the crossover C concentration is much larger and the ferrite melting temperature is much higher than in the real Fe-C alloy. The ABOP of Henriksson and Nordlund without the Ziegler-Biersack-Littmark (ZBL) correction correctly predicts the relative stability of ferrite and austenite at melting, but significantly underestimates the solubility of C in the solid phases, while the same potential with the ZBL correction predicts the austenite to be more stable compared to the ferrite at all C concentrations near the melting transition.

An analytical bond-order potential (BOP) of Fe–Bi has been constructed and has been validated to have a better performance than the Fe–Bi potentials already published in the literature. Molecular dynamics simulations based on this BOP has been then conducted to investigate the ground-state properties of Bi, structural stability of the Fe–Bi binary system, and the effect of Bi on mechanical properties of BCC Fe. It is found that the present BOP could accurately predict the ground-state A7 structure of Bi and its structural parameters, and that a uniform amorphous structure of Fe100−x Bi x could be formed when Bi is located in the composition range of 26 ⩽ x < 70. In addition, simulations also reveal that the addition of a very small percentage of Bi would cause a considerable decrease of tensile strength and critical strain of BCC Fe upon uniaxial tensile loading. The obtained results are in nice agreement with similar experimental observations in the literature.

We develop a new quasi-crystalline approach, combined Pauling Bond Valence-Modified Morse Potential (PBV-MMP), in which the Pauling’s classical relationship between bond valence (BV) and bond length (BL) is incorporated within a modified Morse Potential (MMP) description of the potential energy of interaction between two metal atoms in the bulk, along with the assumption that these interactions are limited to the nearest neighbours with a coordination number . This semi-theoretical approach only needs metal–metal bond energy for a specified valence to provide predictions. For the kinetic and diffusion steps, we utilise Eyring’s transition-state theory and free volume model to estimate entropy changes. The PBV-MMP approach for bulk liquid metals is in the spirit of Unity Bond Index-Quadratic Exponential Potential (UBI-QEP), which has been successful in predicting thermodynamics and kinetics of solid metal surface catalysed reactions. Our model reliably predicts self-diffusivity, viscosity, surface ...

Surface and core formation, thermal and electronic properties of ternary cubic CdZnTe clusters are investigated by using classical molecular dynamics (MD) simulations and density functional theory (DFT) calculations. In this work, MD simulations of the CdZnTe clusters are performed by means of LAMMPS by using bond order potential (BOP). MD simulations are carried out at different temperatures to study the segregation phenomena of Cd, Zn and Te atoms, and deviation of clusters and heat capacity. After that, using optimized geometries obtained, excess charge on atoms, dipole moments, highest occupied molecular orbitals, lowest unoccupied molecular orbitals, HOMO-LUMO gaps ( E g ) , total energies, spin density and the density of states (DOS) have been calculated with DFT. Simulation results such as heat capacity and segregation formation are compared with experimental bulk and theoretical results.

В работе методами молекулярной динамики построена модель нитевидного нановискера (ННК) на основе арсенида галлия (GaAs). Сечение ННК – правильный шестиугольник со стороной 10 нм, высота ННК около 150 нм. ННК закреплен на подложке, при этом ориентация подложки и ННК совпадают и имеют направление (111) вдоль оси роста. Проведено моделирование быстрого термического нагрева ННК до высоких температур. Получены временные зависимости распределения температуры по высоте ННК. Также из модельных расчетов зависимости координат от времени для выделенных атомов в объеме ННК при разных температурах. Показано, что быстрый нагрев возбуждает оптические и акустические фононы. Проведена оценка их энергии. Кроме того, проведен анализ межатомного в разных частях ННК. Построена теоретическая модель для объяснения полученных результатов. Были получены уравнения, дающие оценку критической температуры ННК, при которой он отрывается от основания., In the given work molecular dynamics simulations of a gallium arsenide nanowhisker are realized. During the simulations the nanowhisker was quickly heated. Nanowhisker temperature and atom motion are studied. The results are explained theoretically.

The analytical bond-order potential has been developed for simulating fission product (Ag, Pd, Ru, and I) behavior in SiC, especially for their diffusion. We have proposed adding experimentally available elastic constants and physical properties of the elements as well as important defect formation energies calculated from density functional theory simulation to the list of typical properties as the extensive fitting database. The results from molecular dynamics simulations are in a reasonable agreement with defect properties and energy barriers of their experimental/computational counterparts. The successful validation of the new potential has established a good reliability and transferability of the potentials, which enables the ability of simulation in extended scale. The kinetic behavior such as diffusion of different interstitials is then realized by applying the new interatomic potentials. The bulk diffusion is less likely to dominate the transport of the four fission products under pure thermal condition, when we refer to their extremely small values of the effective diffusion coefficients. The interstitial mechanism is hard for Pd, Ru, and I to access due to the high formation energy and high migration energy. However, it is found that the migration energy of silver is relatively low, which indicates Ag diffusion via an interstitial mechanism being feasible, especially under irradiation condition, where massive interstitials can be formed in high-temperature nuclear reactors.

The stability of the body- and face-centered cubic lattices corresponding to the α and γ phases of Fe, respectively, as well as the transformation of one phase to the other were investigated by atomistic simulations. Two interatomic potentials were used: the embedded atom method (EAM) potential of Meyer and Entel and the bond order potential (BOP) developed by Muller et al. The suitability of the potentials for investigating structural transformations in Fe was verified using nonequilibrium free energy calculations and molecular dynamics simulations. The results showed that the EAM potential is capable of describing the bcc → fcc and fcc → bcc transformations whereas no transformation was observed for the computationally more expensive BOP potential with the simulation set up used.

A fundamental understanding of the inter-relationships between structure, morphology, atomic scale dynamics, chemistry, and physical properties of mixed metallic-covalent systems is essential to design novel functional materials for applications in flexible nano-electronics, energy storage and catalysis. To achieve such knowledge, it is imperative to develop robust and computationally efficient atomistic models that describe atomic interactions accurately within a single framework. Here, we present a unified Tersoff–Brenner type bond order potential (BOP) for a Co–C system, trained against lattice parameters, cohesive energies, equation of state, and elastic constants of different crystalline phases of cobalt as well as orthorhombic Co2C derived from density functional theory (DFT) calculations. The independent BOP parameters are determined using a combination of supervised machine learning (genetic algorithms) and local minimization via the simplex method. Our newly developed BOP accurately describes the structural, thermodynamic, mechanical, and surface properties of both the elemental components as well as the carbide phases, in excellent accordance with DFT calculations and experiments. Using our machine-learnt BOP potential, we performed large-scale molecular dynamics simulations to investigate the effect of metal/carbon concentration on the structure and mechanical properties of porous architectures obtained via self-assembly of cobalt nanoparticles and fullerene molecules. Such porous structures have implications in flexible electronics, where materials with high electrical conductivity and low elastic stiffness are desired. Using unsupervised machine learning (clustering), we identify the pore structure, pore-distribution, and metallic conduction pathways in self-assembled structures at different C/Co ratios. We find that as the C/Co ratio increases, the connectivity between the Co nanoparticles becomes limited, likely resulting in low electrical conductivity; on the other hand, such C-rich hybrid structures are highly flexible (i.e., low stiffness). The BOP model developed in this work is a valuable tool to investigate atomic scale processes, structure–property relationships, and temperature/pressure response of Co–C systems, as well as design organic–inorganic hybrid structures with a desired set of properties.

Accurately predicting bond length and bond dissociation energy for bimetallic diatomic molecules that involve metal–metal multiple bonds is a great challenge for electronic structure theory, in part because many of these molecules have inherently multi-configuration wave functions, a characteristic that is variously labeled as strong correlation or multireference character. Although various popular density functionals are widely used in studying metal–metal bonding in catalysis, their accuracy can be questioned, and it is important to see both how well and how poorly a functional can perform. Here we test 50 Kohn–Sham exchange–correlation density functionals for selected 3d and 4d hetero- and homonuclear bimetallic diatomic molecules against experimental bond lengths and bond energies. We found that for the majority of the density functionals, the mean unsigned error in predicting the bond length is larger than 0.08 A, and for the bond energy, half of the functionals give a mean unsigned error larger than 20 kcal mol−1. This indicates that such highly multireference bimetallic systems are challenging for KS-DFT. However, some exchange–correlation functionals perform significantly better than average for both bond energies and bond lengths, in particular, BLYP, M06-L, N12-SX, OreLYP, RPBE, and revPBE, and are recommended for both kinds of calculations. Other functionals that perform relatively well for bond lengths include MGGA_MS0, MOHLYP, OLYP, PBE, and SOGGA11, and other functionals that perform relatively well for bond energies include GAM, M05, M06, MN15, and τ-HCTHhyb. Although some of these functionals (M05, M06, MN15, N12-SX, and τ-HCTHhyb) contain a nonzero percentage of Hartree–Fock exchange, a broader conclusion is that Hartree–Fock exchange brings in a static correlation error and usually tends to make the results, especially the bond lengths, less accurate. We find some significant differences between all-electron calculations and calculations with effective core potentials. For analysis, the article also presents CASSCF calculations of the percentage contributions of the dominant configurations, and the paper compares orbitals and configurations obtained in DFT calculations to those in CASSCF calculations. The equilibrium bond distance of Rh2 is not available from experiments, and we predict it to be 2.22 A. The bond energy of VCr is not available from experiments, and we predict it to be 52.9 kcal mol−1.

Using Tersoff bond order potential, a vibrational analysis of the spherical fullerene family, namely C60, C80, C180, C240, C260, C320, C500, and C720 was performed. To evaluate the validity of our results, we have compared our simulation results with available experimental data and also with DFT B3LYP/6-31G(d) calculations. In general, molecular stiffness tends to decrease with increasing size, but its variation is limited in cases where mostly the tension-compression interaction sites are active such as the breathing mode. Furthermore, the bond length of each molecule is derived and compared with experimental and theoretical values calculated for graphene. Finally, vibrational frequencies are plotted in a histogram to reveal the common frequency gap and concentration points of the frequency distribution.

We perform a comparative study of six common carbon interatomic potentials: Tersoff, REBO-II, ReaxFF, EDIP, LCBOP-I and COMB3. To ensure fair comparison, all the potentials are used as implemented in the molecular dynamics package LAMMPS. Using the liquid quenching method we generate amorphous carbons at different densities, and subsequently anneal at high temperature. The amorphous carbon system provides a critical test of the transferability of the potential, while the annealing simulations illustrate the graphitization process and test bond-making and -breaking. A wide spread of behavior is seen across the six potentials, with quantities such as sp2 fraction, radial distribution function, morphology, ring statistics, and 002 reflection intensity differing considerably. While none of the potentials is perfect, some perform particularly poorly. The lack of transferability can be traced to the details of the functional form, suggesting future directions in the development of carbon potentials.

We present a new mechanical model of interatomic bonds, which can be used to describe the elastic properties of the carbon allotropes, such as graphite, diamond, fullerene, and carbon nanotubes. The interatomic bond is modeled by a hyperboloid–shape truss structure. The elastic characteristics of this bond are determined. Previous known structural models also used elastic elements (beams, trusses) to simulate a carbon bond. However unlike them our model satisfies to the correct ratio of the longitudinal and lateral stiffness, observed from the previous experimental and theoretical results. Parameters of the bond in application for graphene and diamond were determined.

Al-rich Al1−xCux alloys are important structural materials in the aerospace industry due to their high strength to density ratio. They are also emerging materials for hydrogen containing structures due to their potentially high resistance to hydrogen embrittlement. To enable accurate simulations of the mechanical behavior of Al1−xCux alloys that can guide material improvement, we have developed a high-fidelity analytical bond-order potential (BOP) for the Al-Cu system (the code is publically available in molecular dynamics package LAMMPS). The formalism of the potential is derived from quantum mechanical theories, and the parameters are optimized in an iteration fashion. The iterations begin by fitting properties of a variety of elemental and compound configurations (with coordination varying from 1 to 12) including small clusters, bulk lattices, defects, and surfaces. Following the fitting process, crystalline growth of important equilibrium phases is checked through molecular dynamics simulations of vapor deposition. It is demonstrated that this Al-Cu bond-order potential has unique advantages relative to existing literature potentials in reproducing structural and property tends from experiments and quantum-mechanical calculations, and providing good descriptions of melting temperature, defect characteristics, and surface energies. Most importantly, this BOP is the only potential currently available capable of capturing the Al-rich end of the Al-Cu phase diagram. This capability is rigorously verified by the potential's ability to capture the crystalline growth of the ground-state structures for elemental Al and Cu, as well as, the θ and θ′ phases of the Al2Cu compound in vapor deposition simulations.

The total lifetime distributions for hydrogen bonds in snapshots of molecular dynamics simulations of water serve as a basis to identify a class of proper hydrogen bonds. Proper bonds emerge and break up when restructuring the surrounding area of the hydrogen bond networkwhich weakly depend on the properties of this individual bond, i.e., almost randomly. Therefore, the distribution of the bond lifetimes is described by an exponential function similar to the distribution of the mean free path time in gas. It is shown that proper hydrogen bonds are strong, long-lived, and tetrahedrally oriented bonds. They account for about 80% of the bonds in each snapshot. Thus, these bonds form the basis or framework of the hydrogen bond network of water. The other, improper bonds have a substantially shorter lifetime; these are weak, bifurcated, and quickly switching bonds.

A discussion is carried on about how to characterize quantum mechanically chemical bonds within a molecular structure. Molecular atom- and atom pair-bond operators are constructed. Afterwards an atom and bond expectation values (A-BEV) matrix defined using expectation values of these operators. It is furthermore discussed how one can consider the complete sum of the A-BEV matrix as a collective quantum mechanical descriptor of the atoms and bonds attached to a molecular structure.

In this study, a molecular dynamics (MD) study has been performed on composition and temperature dependences of mechanical properties of CdTe[Formula: see text]Se[Formula: see text] ([Formula: see text], 0.50 and 0.75) nanowires with a diameter of 6.93 nm. The simulation results show that CdTe[Formula: see text]Se[Formula: see text] nanowire seems to be more ductile, whereas CdTe[Formula: see text]Se[Formula: see text] nanowire seems to be more brittle at 1 K. Moreover, the temperature and composition exert significant effects on the mechanical properties of CdTeSe nanowires under stretching. We conclude that the dominancy of Se atoms yields a higher stability and strength at the lower temperature of 1 K, whilst it is the same for the nanowires with both higher Te and Se contents at the higher temperature of 300 K. The radial distribution functions (RDFs) have also been calculated for the CdTeSe nanowires based on the pair separation distance at 1 and 300 K.

Stanene, a quasi-two-dimensional honeycomb-like structure of tin belonging to the family of 2D-Xenes (X = Si, Ge, Sn) has recently been reported to show promising electronic, optical and mechanical properties. This paper investigates the elastic moduli and crack propagation behaviour of single layer and bilayer stanene based on molecular dynamics simulations, which have been performed using the Tersoff bond order potential (BOP). We have parameterized the interlayer van der Waals interactions for the bilayer Lennard-Jones potential in the case of bilayer stanene. Density functional calculations are performed to fit the Lennard-Jones parameters for the properties which are not available from the scientific literature. The effect of temperature and strain rate on the mechanical properties of stanene is investigated for both single layer and bilayer stanene in the armchair and zigzag directions. The results reveal that both the fracture strength and strain of stanene decrease with increasing temperature, while at higher loading rate, the material is found to exhibit higher fracture strength and strain. The effect of chirality on the elastic moduli of stanene is explained on the basis of a physics-based analytical approach, wherein the fundamental interaction between the shear modulus and Young's modulus is elucidated. To provide a realistic perspective, we have investigated the compound effect of uncertainty on the elastic moduli of stanene based on an efficient analytical approach. Large-scale Monte Carlo simulations are carried out considering different degrees of stochasticity. The in-depth results on mechanical properties presented in this article will further aid the adoption of stanene as a potential nano-electro-optical substitute with exciting features such as 2D topological insulating properties with a large bandgap, the capability to support enhanced thermoelectric performance, topological superconductivity and a quantum anomalous Hall effect at near-room-temperature.

International audience; An analytical interatomic bond order potential for the Be–O system is presented. The potential is fitted and compared to a large database of bulk BeO and point defect properties obtained using density functional theory. Its main applications include simulations of plasma-surface interactions involving oxygen or oxide layers on beryllium, as well as simulations of BeO nanotubes and nanosheets. We apply the potential in a study of oxygen irradiation of Be surfaces, and observe the early stages of an oxide layer forming on the Be surface. Predicted thermal and elastic properties of BeO nanotubes and nanosheets are simulated and compared with published ab initio data.

There is a need to choose appropriate interatomic empirical potentials for the molecular dynamics (MD) simulation of nanomachining, so as to represent chip formation and other cutting processes reliably. Popularly applied potentials namely; Lennard-Jones (LJ), Morse, Embedded Atom Method (EAM) and Tersoff were employed in the molecular dynamics simulation of nanometric machining of copper workpiece with diamond tool. The EAM potentials were used for the modelling of the copper-copper atom interactions. The pairs of EAM-Morse and EAM-LJ were used for the workpiece-tool (copper-diamond) atomic interface. The Tersoff potential was used for the carbon-carbon interactions in the diamond tool. Multi-pass simulations were carried out and it was observed that the EAM-LJ and the EAM-Morse pair potentials with the tool modelled as deformable with Tersoff potential were best suitable for the simulation. The former exhibit the lowest cutting forces and the latter has the lowest potential energy.

A molecular dynamics simulations using a recently developed CdZnTe bond order potential is carried out to study structural and thermodynamical properties of the CdZnTe spherical-like ternary nanoparticles with 167–357 atoms in the temperature range 100 K–600 K. The heat capacity calculation is performed as depending the size and the stoichiometry at various temperatures using a non-equilibrated molecular dynamics simulation strategy. Furthermore, the segregation phenomena of Cd, Zn, and Te atoms in the Cd–Zn–Te nanoparticles are investigated by calculating the order parameter R depending on nanoparticle size and temperature. The radial distribution function has also been calculated for the Cd 0.50 Zn 0.50 Te nanoparticle with 357 atoms at 100 K and 600 K.

Hydrogen embrittlement (HE) is a well-known material phenomenon that causes significant loss in the mechanical strength of structural iron and often leads to catastrophic failures. In order to provide a detailed atomistic description of HE we have used a reactive bond order potential to adequately describe the diffusion of hydrogen as well as its chemical interaction with other hydrogen atoms, defects, and the host metal. The currently published ReaxFF force field for Fe/C/H systems was originally developed to describe Fischer-Tropsch (FT) catalysis [C. Zou, A. C. T. van Duin and D. C. Sorescu, Top. Catal., 2012, 55, 391-401], and especially had been trained for surface formation energies, binding energies of small hydrocarbon radicals on different surfaces of iron and the barrier heights of surface reactions. We merged this force field with the latest ReaxFF carbon parameters [S. Goverapet Srinivasan, A. C. T. van Duin and P. Ganesh, J. Phys. Chem. A, 2015, 119, 1089-5639] and used the same training data set to refit the Fe/C interaction parameters. The present work is focused on evaluating the applicability of this reactive force field to describe material characteristics and study the role of defects and impurities in the bulk and at the precipitator interfaces. We study the interactions of hydrogen with pure and defective α-iron (ferrite), Fe3C (cementite), and ferrite-cementite interfaces with a vacancy cluster. We also investigate the growth of nanovoids in α-iron using a grand canonical Monte Carlo (GCMC) scheme. The calculated hydrogen diffusion coefficients for both ferrite and cementite phases predict a decrease in the work of separation with increasing hydrogen concentration at the ferrite-cementite interface, suggesting a hydrogen-induced decohesion behavior. Hydrogen accumulation at the interface was observed during molecular dynamics (MD) simulations, which is consistent with experimental findings. These results demonstrate the ability of the ReaxFF potential to elucidate various aspects of hydrogen embrittlement in α-iron and hydrogen interactions at a more complex metal/metal carbide interface.

Molecular dynamics (MD) simulation was employed in this study to elucidate the dislocation/amorphization-based plasticity mechanisms in single crystal 3C–SiC during nanometric cutting on different crystallographic orientations across a range of cutting temperatures, 300 K to 3000 K, using two sorts of interatomic potentials namely analytical bond order potential (ABOP) and Tersoff potential. Of particular interesting finding while cutting the (110) was the formation and subsequent annihilation of stacking fault-couple and Lomer–Cottrell (L–C) lock at high temperatures, i.e. 2000 K and 3000 K, and generation of the cross-junctions between pairs of counter stacking faults meditated by the gliding of Shockley partials at 3000 K. Another point of interest was the directional dependency of the mode of nanoscale plasticity, i.e. while dislocation nucleation and stacking fault formation were observed to be dominant during cutting the (110), low defect activity was witnessed for the (010) and (111) crystal setups. Nonetheless, the initial response of 3C–SiC substrate was found to be solid-state amorphization for all the studied cases. Further analysis through virtual X-ray diffraction (XRD) and radial distribution function (RDF) showed the crystal quality and structural changes of the substrate during nanometric cutting. A key observation was that the von Mises stress to cause yielding was reduced by 49% on the (110) crystal plane at 3000 K compared to what it took to cut at 300 K. The simulation results were supplemented by additional calculations of mechanical properties, generalized stacking faults energy (GSFE) surfaces and ideal shear stresses for the two main slip systems of 3C–SiC given by the employed interatomic potentials.

Based on the full optimized molecular geometric structures at the B3LYP/cc-pVTZ level, a new designed compound 5-nitro-2-nitratomethyl-1,2,3,4-tetrazole is investigated in order to look for high energy density compounds (HEDCs). The IR spectrum, the heat of formation (HOF), and frontier molecular orbitals are predicted. The detonation velocity and pressure are evaluated using Kamlet–Jacobs equations based on the theoretical density and condensed HOF. The bond dissociation energies (BDEs) and bond orders for the weakest bonds are analyzed to investigate the thermal stability of the title compound. The results show that the O1–N6 bond is the trigger bond. The crystal structure obtained by molecular mechanics belongs to the Pna21 space group with lattice parameters Z = 4, a = 13.7565 A, b = 12.4737 A, c = 4.3445 A.

The theoretical description of the thermally activated dislocation glide in pure crystals depends crucially on the shape of the Peierls barrier that the dislocation has to overcome when moving through the lattice. While the height of this barrier can be obtained using saddle-point search algorithms such as the Nudged Elastic Band (NEB) method, its exact shape depends on the details of the approximation of the transition pathway of the system. The purpose of this paper is to formulate a procedure that allows to identify the dislocation positions along the path directly from the displacements of atoms in its core. We investigate the performance of this model by calculating curved paths of the 1 / 2 [ 111 ] screw dislocation in tungsten modeled by a Bond Order Potential using a series of images obtained by employing a modified NEB method at zero applied stress and for positive/negative shear stresses perpendicular to the slip direction. The Peierls barriers resulting from the curved paths are shown to be substantially different from those obtained when assuming straight dislocation path. For both straight and curved dislocation pathways we calculate the temperature dependencies of the flow stress and compare these predictions with direct experimental measurements. We show that a significantly better agreement with experiments is obtained if the curved dislocation pathway is taken into account.

Purpose – The purpose of this paper is to study elastic properties of III-V nitride nanotubes (NNTs) using second generation (REBO) potential. Design/methodology/approach – In the present research paper elastic properties of BN, AlN and GaN nanotubes have been investigated, using the second generation REBO potential by Brenner and co-workers, which is a bond order potential earlier used for carbon nanostructures successfully. In the present calculation, the same form of potential is used with adjusted parameters for h-BN, h-AlN and h-GaN. In all these cases the authors have considered graphite like network and strongly polar nature of these atoms so electrostatic forces are expected to play an important role in determining elastic properties of these nanotubes. The authors generate the coordinates of nanotubes of different chirality’s and size. Each and every structure thus generated is allowed to relax till the authors obtain minima of energy. The authors then apply the requisite compressions, elongations and twists to the structures and compute the elastic moduli. Young’s Modulus, Shear Modulus and Poisson’s ratio for single-walled armchair and zigzag tubes of different chirality’s and size have been calculated. The computational results show the variation of Young’s Modulus, Poisson’s ratio and Shear Modulus for these NNTs with nanotube diameter. The results have been compared with available data, experimental as well as theoretical. Findings – The authors have calculated bond length, cohesive energy/bond, Strain energy, Young’s Modulus, Shear Modulus and Poisson’s ratio. Originality/value – To the best of the knowledge this work is the first attempt to study elastic properties of III-V NNTs using second generation REBO potential

Defects in multilayered films have long been a performance-limiting problem for the semiconductor industry. For instance, CdTe/CdS solar cell efficiencies have had significant improvement in the past 15years or more without addressing the problem of high misfit dislocation densities. Overcoming this stagnation requires a fundamental understanding of interfacial defect formation. Herein, we demonstrate a new first principles-based CdTe bond-order approach that enables efficient molecular dynamics to approach the fidelity of density functional theory. Stringent quantum-mechanical verification and experimental validation tests reveal that our new approach provides an accurate prediction of defects that earlier methods cannot. Using this new capability, we show that misfit dislocations in CdTe/CdS multilayers can be significantly reduced via nano-patterning and composition grading and more importantly, dislocation-free multilayers naturally arise when the pattern dimension is reduced below 90nm. Our predictive methods are generally applicable to other materials, highlighting a rational approach towards low-defect semiconductor films. Copyright © 2015 John Wiley & Sons, Ltd.

WOS: 000441595000011 In this study, the geometrical, thermal, and energetic properties of zinc-blende CdTe1-xSex (x = 0.25, 0.50, and 0.75) nanoparticles were investigated using the bond order potential based on the modern classical molecular dynamics (MD) method. All MD simulations were performed using LAMMPS. Some physical properties were investigated, such as compositional variations of Cd, Te, and Se atoms; order parameter; radial distribution function; coordination number; potential energy; and heat capacity (Cv). The simulation results were compared with the available experimental results. The obtained results revealed that an increase in the composition of Se atoms can provide contributions to stability, which is desirable to increase the efficiency of solar cells. Ahi Evran University Scientific Research Projects Coordination UnitAhi Evran University [TBY.C1.17.001] This work was supported by the Ahi Evran University Scientific Research Projects Coordination Unit ( Project Number: TBY.C1.17.001). The numerical calculations reported in this paper were partially performed at the TUBITAK ULAKBIM High Performance and Grid Computing Center (TRUBA resources).

The hydrogen behavior in nanocrystalline W (ncW) samples with grain size of 5 and 10 nm is studied using Molecular Dynamics (MD) with a bond order potential (BOP) for the W-H system. The dependence of the hydrogen diffusion coefficient on grain size (5 and 10 nm) and hydrogen concentration (0.1 at.% < [H] < 10.0 at.%) is calculated. These data show that in all cases the hydrogen diffusion coefficient is lower for ncW than for coarse-grained samples. Trapping energies of grain boundaries are estimated and a broad distribution roughly centered at the vacancy trapping energy is found. Hydrogen diffusion results are interpreted within the trapping model by Kirchheim for nanocrystalline materials. The H-H interaction is evaluated and the possible formation of H2 is disregarded for the conditions in these simulations. Hydrogen segregation and trapping in grain boundaries for ncW is discussed, including extrapolations for micron-sized polycrystals. Fil: Piaggi, Pablo M.. Universidad Nacional de San Martín. Instituto Sabato; Argentina Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Pasianot, Roberto Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Gordillo, Nuria. Universidad Politécnica de Madrid; España Fil: Panizo Laiz, M.. Universidad Politécnica de Madrid; España Fil: Del Río, J.. Universidad Complutense de Madrid; España Fil: Gómez De Castro, C.. Universidad Complutense de Madrid; España Fil: Gonzalez Arrabal, R.. Universidad Politécnica de Madrid; España

Despite extensive studies in the past, deterioration of mechanical properties due to hydrogen environment exposure remains a serious problem for structural materials. More effective improvement of a material’s resilience requires advanced computational methods to elucidate the fundamental mechanisms of the hydrogen effects. To enable accurate molecular dynamics (MD) studies of the hydrogen effects on metals, we have developed a high-fidelity analytical bond-order potential (BOP) for the copper–hydrogen binary system as a representative case. This potential is available through the publically available MD code LAMMPS. The potential parameters are optimized using an iterative process. First, the potential is fitted to static and reactive properties of a variety of elemental and binary configurations including small clusters and bulk lattices (with coordination varying from 1 to 12). Then the potential is put through a series of rigorous MD simulation tests (e.g., vapor deposition and solidification) that involve chaotic initial configurations. It is demonstrated that this Cu–H BOP not only gives structural and property trends close to those seen in experiments and quantum mechanical calculations, but also predicts the correct phase transformations and chemical reactions in direct MD simulations. The correct structural evolution from chaotic initial states strongly verifies the transferability of the potential. A highly transferable potential is the reason that a well-parameterized analytical BOP can enable MD simulations of metal-hydrogen interactions to reach a fidelity level not achieved in the past.

© the Owner Societies 2017. The concept of chemical bonding is normally presented and simplified through two models: the covalent bond and the ionic bond. Expansion of the ideal covalent and ionic models leads chemists to the concepts of electronegativity and polarizability, and thus to the classification of polar and non-polar bonds. In addition, the intermolecular interactions are normally viewed as physical phenomena without direct correlation to the chemical bond in any simplistic model. Contrary to these traditional concepts of chemical bonding, recently developed canonical approaches demonstrate a unified perspective on the nature of binding in pairwise interatomic interactions. This new canonical model, which is a force-based approach with a basis in fundamental molecular quantum mechanics, confirms much earlier assertions that in fact there are no fundamental distinctions among covalent bonds, ionic bonds, and intermolecular interactions including the hydrogen bond, the halogen bond, and van der Waals interactions.

The history and theory of one- and three-electron bonds are presented and discussed. It is shown that the dependence of the odd-electron bond energy on the difference in ionization potentials of the bonding partners must be corrected to include the ion-pairing energy for neutral odd-electron bonded complexes.

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid. This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.

The bond-valence model (BVM) posits an inverse relationship between bond valence (essentially bond order) and bond length, typically described by either exponential or power-law equations. To assess the value of these forms for describing a wider range of bond lengths than found in crystals, we first assume that the bond critical point density (ρ b , reported in e − /A 3 ) is at least roughly proportional to bond valence. We then calculate ρ b -distance curves for several diatomic pairs using electronic structure calculations (CCSD/aug-cc-pVQZ) and Atoms-In-Molecules (AIM) analysis. The shapes of these curves cannot be completely described by the standard exponential and power-law forms, but are well described by a three-parameter hybrid of the exponential and power-law forms. The ρ b -distance curves for covalent bonds tend to exhibit exponential behavior, while metallic bonds exhibit power-law behavior, and ionic bonds tend to exhibit a combination of the two. We next use a suite of both experimental and calculated (B3LYP/Def2-TZVP) molecular structures of oxo-molecules, for which we could infer X-O bond valences of ~1 or ~2 v.u., combined with some crystal structure data, to estimate the curvature of the bond valence-length relationship in the high-valence region. Consistent with the results for the ρ b -distance curves, the standard forms of the bond valence-length equation become inadequate to describe high-valence bonds as they become more ionic. However, some of these systems demonstrate even higher curvature changes than our three-parameter hybrid form can manage. Therefore, we introduce a four-parameter hybrid form, and discuss possible reasons for the severe curvature. Although the addition of more parameters to the bond valence-length equation comes at a cost in terms of model simplicity and ease of optimization, they will be necessary to make the BVM useful for molecular systems and transition states.

According to the Pauling bond orders, the lengths of the rung carbon–carbon bonds along the central hexagonal chain in chevron-type benzenoid molecules are monotonically changed. The calculated bond lengths obtained at the B3LYP/6-311G(d,p) level of theory show that this regularity holds only for the first few members of the chevron homologous series. In the case of higher members of the series, the predictions of the Pauling bond orders are false. This indicates that the Pauling bond orders are not generally applicable for the prediction of bond lengths.