Reactive Empirical Bond Order Potential Research Articles

Pressurized CNTs under tension: A finite-deformation lattice model

We propose a finite-deformation lattice model for carbon nanotubes (CNTs), in which an energetic cost is assigned to changes in bond lengths, bond angles, and dihedral angles, while atomic interactions are governed by a Reactive Empirical Bond-Order potential. For the first time, we consider the effect of severe traction and pressure applied simultaneously, with the ultimate goal of understanding how material properties are affected by such loads. In particular, the radial stiffness and the axial stiffness have been investigated: on making use of the second generation Brenner potential, we have found that peculiar transitions softening-hardening-softening occur in both the radial and axial stiffness, a behavior that could be compared with the ones observed in experiments or predicted by ab initio methods. The constitutive response described here can be exploited in the design of CNT-based hydrogen storage systems, and it opens a new possibility for tuning material properties, in order to make CNTs softer or harder in a controlled way.

The Effect of VMoS3 Point Defect on the Elastic Properties of Monolayer MoS2 with REBO Potentials

Structural defects in monolayer molybdenum disulfide (MoS2) have significant influence on the electric, optical, thermal, chemical, and mechanical properties of the material. Among all the types of structural defects of the chemical vapor phase-grown monolayer MoS2, the VMoS3 point defect (a vacancy complex of Mo and three nearby S atoms) is another type of defect preferentially generated by the extended electron irradiation. Here, using the classical molecular dynamics simulation with reactive empirical bond-order (REBO) potential, we first investigate the effect of VMoS3 point defects on the elastic properties of monolayer MoS2 sheets. Under the constrained uniaxial tensile test, the elastic properties of monolayer MoS2 sheets containing VMoS3 vacancies with defect fraction varying from 0.01 to 0.1 are obtained based on the plane anisotropic constitutive relations of the material. It is found that the increase of VMoS3 vacancy concentration leads to the noticeable decrease in the elastic modulus but has a slight effect on Poisson’s ratio. The maximum decrease of the elastic modulus is up to 25 %. Increasing the ambient temperature from 10 K to 500 K has trivial influences on the elastic modulus and Poisson’s ratio for the monolayer MoS2 without defect and with 5 % VMoS3 vacancies. However, an anomalous parabolic relationship between the elastic modulus and the temperature is found in the monolayer MoS2 containing 0.1 % VMoS3 vacancy, bringing a crucial and fundamental issue to the application of monolayer MoS2 with defects.

Elucidating the Properties of Surrogate Fuel Mixtures Using Molecular Dynamics

The wide compositional differences between conventional and alternative fuels have resulted in much research aimed at determining which alternative fuels can be used, and in what proportions, in conventional engines. Atomic-scale modeling is uniquely positioned to lend insight into this question without extensive large-scale tests. The predictive power such modeling affords could narrow the phase space that must be examined experimentally. This study utilizes molecular dynamics (MD) simulations to predict the properties of a set of pure hydrocarbons, as well as binary and multicomponent surrogate fuel mixtures for alternative fuels created from these pure components. The accuracy and transferability of the modified Lennard-Jones adaptive intermolecular reactive empirical bond-order potential (mod-LJ AIREBO) [Liu, A.; Stuart, S. J. J. Comput. Chem. 2008, 29, 601−611] was assessed by calculating densities, heats of vaporization, and bulk moduli of pure hydrocarbons and the mixtures of these hydrocarbons, i.e., surrogate fuels. Calculated results were compared to experimentally determined values and to values obtained with the nonreactive, all-atom version of the optimized potential for liquid simulations (OPLS-AA) [Jorgensen, W. L.; Maxwell, D. S.; TiradoRives, J. J. Am. Chem. Soc. 1996, 118, 11225−11236]. The mod-LJ AIREBO potential quantitatively predicts the densities of the pure hydrocarbons and binary mixtures of n-dodecane and 2,2,4,4,6,8,8-heptamethylnonane (isocetane). It is interesting to note, that despite doing an excellent job predicting the densities of the pure hydrocarbons, the performance of the mod-LJ AIREBO potential degrades when predicting the densities of the multicomponent surrogates and mixtures of n-hexadecane and isocetane, implying that it is not straightforward to extend potentials fit with pure compounds to mixtures. The OPLS-AA potential also has difficulty quantitatively predicting the densities of mixtures, although a new parameter set for long-chain hydrocarbons (L-OPLS) [Siu, S. W. I.; Pluhackova, K.; Bockmann, R. A. J. Chem. Theory Comput. 2012, 8, 1459−1470] yields some improvement for binary mixtures. Heat of vaporization predictions using both potentials also agree reasonably well with experiment. Bulk moduli predictions using the mod-LJ AIREBO potential are consistently higher than, and do not quantitatively agree with, the experimental values. In contrast, bulk moduli predictions using the OPLS-AA potential are generally in good agreement with experimental values. Despite the success of the OPLS-AA potential predicting the bulk moduli of individual hydrocarbons, it is unable to quantitatively predict the bulk moduli of the multicomponent surrogates. Interestingly, the use of the L-OPLS parameter set improves density predictions but not predicted bulk moduli values.

Bending response of single layer MoS2

Using molecular mechanics (or dynamics) simulations, three different approaches, including the targeted molecular mechanics, four-point bending and nanotube methods, are employed to investigate the bending response of single layer MoS2 (SLMoS2), among which four-point bending is the most accurate approach to determine the bending stiffness according to the continuum theory. It is found that when the bending curvature radius is large enough (e.g. >4 nm), three approaches will give the same bending stiffness of SLMoS2 and the bending behavior is isotropic for SLMoS2, whereas the nanotube method with small tubes (e.g. <4 nm) cannot give the correct bending stiffness. Compared with the reported result from the MoS2 nanotube calculated by density functional theory, the revised Stillinger–Weber (SW) and reactive empirical bond-order (REBO) potentials can give the reasonable bending stiffness of SLMoS2 (8.7–13.4 eV) as well as the effective deformed conformation. In addition, since the Mo–S bond deformation of SLMoS2 under bending is similar to that under in-plane tension/compression, the continuum bending theory can quite accurately predict the bending stiffness of SLMoS2 if a reasonable thickness of SLMoS2 is given. For SLMoS2, the reasonable thickness should be larger than the distance between its two S atomic planes and lower than the distance between two Mo atomic planes of bulk MoS2 crystal, e.g. 0.375–0.445 nm.

Geometry and Self-stress of Single-Wall Carbon Nanotubes and Graphene via a Discrete Model Based on a 2nd-Generation REBO Potential

The main purpose of this paper is to evaluate the self-stress state of single-walled Carbon NanoTubes (CNTs) and Flat Graphene Strips (FGSs) in their natural equilibrium state, that is, the state prior to the application of external loads. We model CNTs as discrete elastic structures, whose shape and volume changes are governed by a Reactive Empirical Bond-Order (REBO) interatomic potential of second generation. The kinematical variables we consider are bond lengths, bond angles, and dihedral angles; to changes of each of these variables we associate a work-conjugate nanostress. To determine the self-stress state in a given CNT, we formulate the load-free equilibrium problem as a minimum problem for the interatomic potential, whose solution yields the equilibrium nanostresses; next, by exploiting the nonlinear constitutive dependence we derive for nanostresses in terms of a list of kinematical variables, we determine the equilibrium values of the latter; finally, from the equilibrium values of the kinematical variables we deduce the natural geometry and, in particular, the natural radius. Our theoretical framework accommodates CNTs of whatever chirality. In the achiral case, the stationarity conditions implied by energy minimization are relatively easy to derive and solve numerically, because we can count on maximal intrinsic symmetries and hence the number of independent unknowns is reduced to a minimum; for chiral CNTs, we prefer to solve the minimum problem directly. The natural-radius predictions we achieve within our discrete-mechanics framework are in good agreement with the results of calculations based on Density Functional Theory (DFT) and Tight Binding (TB) theory; the same is true for our predictions of the self-energy, that is, the energy associated with self-stress (called cohesive energy in the literature); we surmise that our discrete mechanical model may serve as a source of benchmarks for Molecular-Dynamics (MD) simulation algorithms. We find that self-stress depends on changes in both bond and dihedral angles in achiral CNTs and, in addition, on changes in bond length in chiral CNTs. Our analysis applies also to FGSs, whose self-stress and self-energy we evaluate; we find that in FGSs self-stress is associated exclusively with changes in bond angle.

Influence of helical rise on the self-excited oscillation behavior of zigzag @ zigzag double-wall carbon nanotubes

Oscillation behaviors of oscillators consisting of defect-free multi-walled carbon nanotubes (MWCNTs) have been extensively studied, owing to the operating frequency of the nanotubes being able to reach up to gigahertz. However, there exist defects in most carbon nanotubes, which will affect the friction force between the walls of nanotubes. It is therefore critical to investigate the oscillation characteristics of the MWCNT-based oscillators containing a distorted or defective rotating tube, for the design of MWCNTs-based oscillators. Unlike the case in the armchair carbon nanotubes (Zeng Y H, et al. 2016 Nanotechnology 27 95705), the existence of the helical rise in the zigzag-type nanotubes can induce aberrant or defective shell structures. In this paper, the oscillatory behaviors of zigzag@zigzag double-wall carbon nanotubes containing a rotating inner tube with different helical rises are investigated using the molecular dynamics method. In all the simulation modes, the adaptive intermolecular reactive empirical bond order potential is used in this work for both the covalent bond between carbon atoms and the long-range van der Waals interaction of the force field. The perfect zigzag outer tube is assumed to be fixed while the zigzag inner tube is free after it has been rotated by a torque. At the beginning of the simulation, the whole system is heat bathed at a temperature around 300 K for 60 ps, to gently increase the whole system temperature to around 300 K after the energy minimization. The total number of particles, the system volume, and the absolute temperature are kept unchanged for 60 ps. Then we apply a torque of 30 eV to the inner tube under the constant temperature. After the rotation frequency of the inner tube reaches around 300 GHz, we remove the torque of inner tube and let the whole system be under a constant energy condition. The time steps for all simulations are all chosen to be 1 fs. The total time for the simulation is 3000 ps. It is found that the oscillatory behavior of the inner tube is dependent on the helical rise. The simulation results show that the oscillation frequency of the inner tube increases with the length of helical rise increasing. However, as the helical rise is further increased, the oscillation becomes awful because of the breakage of the inner tube with defects. Moreover, the zigzag@zigzag double-wall carbon nanotubes without any helical rise may be used as an ideal rotating actuator because the inner tube can rotate at an approximately constant rotational frequency. The influence of the system temperature on the oscillatory behavior of inner tube with a helical rise of 0.5 nm is also investigated. The results show that the oscillation amplitude of the inner tube increases with temperature increasing, but the oscillation of the inner tube is extremely unstable if the temperature is higher than a critical value.

Classical molecular dynamics simulation on the dynamical properties of H2on silicene layer

This study investigates the diffusion of hydrogen molecule physisorbed on the surface of silicene nanoribbon (SiNR)using the classical molecular dynamic (MD) simulation in LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). The interactions between silicon atoms are modeled using the modified Tersoff potential, the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential for hydrogen – hydrogen interaction and the Lennard – Jones potential for the physisorbed H2 on SiNR. By varying the temperatures (60 K Δ 130 K), we observed that the Δxdisplacement of H 2 on the surface SiNR shows a Brownian motion on a Lennard-Jones potential and a Gaussian probability distribution can be plotted describing the diffusion of H 2 . The calculated mean square displacement (MSD) was approximately increasing in time and the activation energy barrier for diffusion has been found to be 43.23meV.

Equilibrium Molecular Dynamics (MD) Simulation Study of Thermal Conductivity of Graphene Nanoribbon: A Comparative Study on MD Potentials

The thermal conductivity of graphene nanoribbons (GNRs) has been investigated using equilibrium molecular dynamics (EMD) simulation based on Green-Kubo (GK) method to compare two interatomic potentials namely optimized Tersoff and 2nd generation Reactive Empirical Bond Order (REBO). Our comparative study includes the estimation of thermal conductivity as a function of temperature, length and width of GNR for both the potentials. The thermal conductivity of graphene nanoribbon decreases with the increase of temperature. Quantum correction has been introduced for thermal conductivity as a function of temperature to include quantum effect below Debye temperature. Our results show that for temperatures up to Debye temperature, thermal conductivity increases, attains its peak and then falls off monotonically. Thermal conductivity is found to decrease with the increasing length for optimized Tersoff potential. However, thermal conductivity has been reported to increase with length using 2nd generation REBO potential for the GNRs of same size. Thermal conductivity, for the specified range of width, demonstrates an increasing trend with the increase of width for both the concerned potentials. In comparison with 2nd generation REBO potential, optimized Tersoff potential demonstrates a better modeling of thermal conductivity as well as provides a more appropriate description of phonon thermal transport in graphene nanoribbon. Such comparative study would provide a good insight for the optimization of the thermal conductivity of graphene nanoribbons under diverse conditions.

Mechanical behavior of carbon nanotube and graphene junction as a building block for 3D carbon nanostructures

The incorporation of defects in junction area of 1D and 2D carbon nanostructures has a major impact on properties of their 3D structures. In the present study, molecular dynamics simulation is utilized to examine the mechanical behavior of graphene sheet (GS) in carbon nanotube (CNT)-GS junctions. The tensile load was applied along the GS in connection with CNTs of different chiralities. The adaptive intermolecular reactive empirical bond order potential was chosen to model C-C interactions. It provided a reliable model for CNT, GS and their junctions. The results revealed that the connection of CNT to the GS with a hole could improve the mechanical properties of defective GS, which appeared to be independent of CNT type. It was found that the high strength C-C bonds postpone the crack propagation and motivates new crack nucleation. When a hole or CNT placed on the GS, it caused stress concentration, exactly along a line on its side. The lower mechanical properties were consequently associated with crack nucleation and propagation on both sides in a way that cracks encountered each other during the failure; while, the cracks in pristine GS propagate parallel to each other and could not encounter each other.

Atomistic simulation of detonation initiation by ultra-short impact

We present results of the classical molecular dynamics simulation of detonation initiation in simple AB model of a high explosive compressed by ultra-short shock wave (SW). The simplified reactive empirical bond order potential (REBO) defines interatomic forces in the AB model explosive made up of diatomic AB molecules. Simulation of ultra-short piston-driven compression of AB explosive with duration of picoseconds represents an indirect initiation via a thin metal foil irradiated by a femtosecond laser pulse. We studied transition of SW to a detonation wave (DW), including evolution of calculated pressure profile in a sample. A run distance to detonation of such AB explosive film, which is required for detonation initiation, was obtained. Variation of loading time and piston velocity gives a 2D region of transition from SW to DW. The influence of pores on detonation initiation threshold is discussed.

Recent developments and simulations utilizing bond-order potentials

Bond-order potentials (BOPs) have been used successfully in simulations of a wide range of processes. A brief overview of bond-order potentials is provided which focuses on the reactive empirical bond-order (REBO) potential for hydrocarbons (Brenner et al 2002 J. Phys.: Condens. Matter 14 783) and the large number of useful potentials it has spawned. Two specific extensions of the REBO potential that make use of its formalism are discussed. First, the 2B-SiCH potential (Schall and Harrison 2013 J. Phys. Chem. C 117 1323) makes the appropriate changes to the hydrocarbon REBO potential so that three atom types, Si, C, and H, can be modeled. Second, we recently added the electronegative element O, along with the associated charge terms, to the adaptive intermolecular REBO (AIREBO) potential (Stuart et al 2000 J. Chem. Phys. 112 6472). The resulting qAIREBO potential (Knippenberg et al 2012 J. Chem. Phys. 136 164701) makes use of the bond-order potential/split-charge (BOP/SQE) equilibration method (Mikulski et al 2009 J. Chem. Phys. 131 241105) and the Lagrangian approach to charge dynamics (Rick et al 1994 J. Chem. Phys. 101 6141). The integration of these two techniques allows for atomic charges to evolve with time during MD simulations: as a result, chemical reactions can be modeled in C-, O-, and H-containing systems. The usefulness of the 2B-SiCH potential for tribological investigations is demonstrated in molecular dynamics (MD) simulations of axisymmetric tips composed of Si and SiC placed in sliding contact with diamond(1 1 1) surfaces with varying amounts of hydrogen termination. The qAIREBO potential is used to investigate confinement of sub-monolayer coverages of water between nanostructured surfaces.

The atomic scale structure of graphene powder studied by neutron and X-ray diffraction

The structure of graphene obtained by chemical exfoliation of graphiteviathe oxidation/reduction procedure has been determined using wide-angle scattering of neutrons and X-rays combined with computer simulations based on classical molecular dynamics (MD). A comparison of results obtained from wide-angle neutron scattering (WANS) with the D4 neutron diffractometer dedicated for liquids and amorphous materials (Institute Laue–Langevin in Grenoble) and from wide-angle X-ray scattering (WAXS) with the laboratory Rigaku-Denki D/MAX RAPID II diffractometer has shown that both techniques provide data of a good quality that can be used to derive precise and valuable structural information about graphene. To obtain detailed structural information, the paracrystal formalism has been used along with MD simulations. The MD simulations were performed at 300 K with second-generation reactive empirical bond order potential for atoms lying in the same layer and the Lennard–Jones potential for interlayer interactions. The proposed models consist of three-layered systems, 36 Å in diameter, in which mono-vacancy, di-vacancy and Stone–Thrower–Wales types of defects are introduced. The reported results show that the WANS and WAXS methods together with the MD simulations contribute to a detailed description of the graphene materials, including the presence of topological defects, which is important as their structure at the atomic scale dramatically affects their electrical and mechanical properties.

Graphene as a hexagonal 2-lattice: Evaluation of the in-plane material constants for the linear theory. A multiscale approach

Continuum modeling of free-standing graphene monolayer, viewed as a two dimensional 2-lattice, requires specification of the components of the shift vector that acts as an auxiliary variable. If only in-plane motions are considered, the energy depends on an in-plane strain measure and the shift vector. The assumption of geometrical and material linearity leads to quadratic energy terms with respect to the shift vector, the strain tensor, and their combinations. Graphene's hexagonal symmetry reduces the number of independent moduli then to four. We evaluate these four material parameters using molecular calculations and the adaptive intermolecular reactive empirical bond order potential and compare them with standard linear elastic constitutive modeling. The results of our calculations show that the predicted values are in reasonable agreement with those obtained solely from our molecular calculations as well as those from the literature. To the best of our knowledge, this is the first attempt to measure mechanical properties when graphene is modeled as a hexagonal 2-lattice. This work targets at the continuum scale when the insight measurements come from finer scales using atomistic simulations.

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

In this work, the ability of methods based on empirical potentials to simulate the effects of radiation damage in graphite is examined by comparing results for point defects, found using ab initio calculations based on density functional theory (DFT), with those given by two state of the art potentials: the Environment-Dependent Interatomic Potential (EDIP) and the Adaptive Intermolecular Reactive Empirical Bond Order potential (AIREBO). Formation energies for the interstitial, the vacancy and the Stone–Wales (5775) defect are all reasonably close to DFT values. Both EDIP and AIREBO can thus be suitable for the prompt defects in a cascade, for example. Both potentials suffer from arefacts. One is the pinch defect, where two α-atoms adopt a fourfold-coordinated sp3 configuration, that forms a cross-link between neighbouring graphene sheets. Another, for AIREBO only, is that its ground state vacancy structure is close to the transition state found by DFT for migration. The EDIP fails to reproduce the ground state self-interstitial structure given by DFT, but has nearly the same formation energy. Also, for both potentials, the energy barriers that control diffusion and the evolution of a damage cascade, are not well reproduced. In particular the EDIP gives a barrier to removal of the Stone–Wales defect as 0.9 eV against DFT's 4.5 eV. The suite of defect structures used is provided as supplementary information as a benchmark set for future potentials.

Multiscale modeling of functionalized interface effects on the effective elastic material properties of CNT–polyethylene nanocomposites

The effects of functionalization of the interface between carbon nanotubes (CNTs) and the polymer matrix on the macroscale elastic mechanical material properties have been investigated using a multiscale model. Molecular dynamic (MD) simulations are used to investigate the influence of functionalization on the nanoscale load transfer ability of the interface between the CNT and the polymer matrix material in CNT–polyethylene (PE) nanocomposites. Graphene–polymer interface models with variable numbers of functional groups grafted between the graphene sheet and the polymer chains are adopted to represent the interface between the CNTs and the polymer matrix. Normal mode and sliding mode separations are performed using MD simulations in order to characterize the load transfer of the functionalized graphene–polymer interface in terms of force-separation responses. The influence of the functionalization density on the load transfer at the interface is studied parametrically. Two force field potentials, the consistent valence force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potential, are used to simulate the interactions between the atoms in the polymer matrix and the functional groups. An energy based bond breaking criteria is introduced into the CVFF potential such that both potentials allow for bond breaking. Comparison of the two potentials is conducted by comparing the MD simulation results where it is found that two potentials give different force-separation responses, but that in both cases, there is significant increase in load transfer capability and toughness of the interface with increasing degree of functionalization of the interface. With cohesive zone laws derived from MD results, a finite element implementation of the cohesive zone models is used to calculate the effective elastic mechanical properties of the CNT–PE nanocomposites in order to study the influence of functionalization at the nanoscale on their macroscale mechanical performance.

Elastic moduli of boron nitride, aluminium nitride and gallium nitride nanotubes using second generation reactive empirical bond order potential

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

On the prediction of graphene’s elastic properties with reactive empirical bond order potentials

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively.

Graphene-Based Resonant Sensors for Detection of Ultra-Fine Nanoparticles: Molecular Dynamics and Nonlocal Elasticity Investigations

Application of single layered graphene sheets (SLGSs) as resonant sensors in detection of ultra-fine nanoparticles (NPs) is investigated via molecular dynamics (MD) and nonlocal elasticity approaches. To take into consideration the effect of geometric nonlinearity, nonlocality and atomic interactions between SLGSs and NPs, a nonlinear nonlocal plate model carrying an attached mass-spring system is introduced and a combination of pseudo-spectral (PS) and integral quadrature (IQ) methods is proposed to numerically determine the frequency shifts caused by the attached metal NPs. In MD simulations, interactions between carbon–carbon, metal–metal and metal–carbon atoms are described by adaptive intermolecular reactive empirical bond order (AIREBO) potential, embedded atom method (EAM), and Lennard–Jones (L–J) potential, respectively. Nonlocal small-scale parameter is calibrated by matching frequency shifts obtained by nonlocal and MD simulation approaches with same vibration amplitude. The influence of nonlinearity, nonlocality and distribution of attached NPs on frequency shifts and sensitivity of the SLGS sensors are discussed in detail.

Combining a reactive potential with a harmonic approximation for molecular dynamics simulation of failure: construction of a reduced potential

Molecular dynamics is a simulation technique that can be used to study failure in solids, provided the inter-atomic potential energy is able to account for the complex mechanisms at failure. Reactive potentials fitted on ab initio results or on experimental values have the ability to adapt to any complex atomic arrangement and, therefore, are suited to simulate failure. But the complexity of these potentials, together with the size of the systems considered, make simulations computationally expensive. In order to improve the efficiency of numerical simulations, simpler harmonic potentials can be used instead of complex reactive potentials in the regions where the system is close to its ground state and a harmonic approximation reasonably fits the actual reactive potential. However the validity and precision of such an approach has not been investigated in detail yet.We present here a methodology for constructing a reduced potential and combining it with the reactive one. We also report some important features of crack propagation that may be affected by the coupling of reactive and reduced potentials. As an illustrative case, we model a crystalline two-dimensional material (graphene) with a reactive empirical bond-order potential (REBO) or with harmonic potentials made of bond and angle springs that are designed to reproduce the second order approximation of REBO in the ground state. We analyze the consistency of this approximation by comparing the mechanical behavior and the phonon spectra of systems modeled with these potentials. These tests reveal when the anharmonicity effects appear. As anharmonic effects originate from strain, stress or temperature, the latter quantities are the basis for establishing coupling criteria for on the fly substitution in large simulations.

AIREBO-M: a reactive model for hydrocarbons at extreme pressures.

The Adaptive Intermolecular Reactive Empirical Bond Order potential (AIREBO) for hydrocarbons has been widely used to study dynamic bonding processes under ambient conditions. However, its intermolecular interactions are modeled by a Lennard-Jones (LJ) potential whose unphysically divergent power-law repulsion causes AIREBO to fail when applied to systems at high pressure. We present a modified potential, AIREBO-M, where we have replaced the singular Lennard-Jones potential with a Morse potential. We optimize the new functional form to improve intermolecular steric repulsions, while preserving the ambient thermodynamics of the original potentials as much as possible. The potential is fit to experimental measurements of the layer spacing of graphite up to 14 GPa and first principles calculations of steric interactions between small alkanes. To validate AIREBO-M's accuracy and transferability, we apply it to a graphite bilayer and orthorhombic polyethylene. AIREBO-M gives bilayer compression consistent with quantum calculations, and it accurately reproduces the quasistatic and shock compression of orthorhombic polyethlyene up to at least 40 GPa.