Showing papers on "Interatomic potential published in 2017"

Machine learning based interatomic potential for amorphous carbon

Abstract: We introduce a Gaussian approximation potential (GAP) for atomistic simulations of liquid and amorphous elemental carbon. Based on a machine learning representation of the density-functional theory (DFT) potential-energy surface, such interatomic potentials enable materials simulations with close-to DFT accuracy but at much lower computational cost. We first determine the maximum accuracy that any finite-range potential can achieve in carbon structures; then, using a hierarchical set of two-, three-, and many-body structural descriptors, we construct a GAP model that can indeed reach the target accuracy. The potential yields accurate energetic and structural properties over a wide range of densities; it also correctly captures the structure of the liquid phases, at variance with a state-of-the-art empirical potential. Exemplary applications of the GAP model to surfaces of ``diamondlike'' tetrahedral amorphous carbon ($\mathit{ta}$-C) are presented, including an estimate of the amorphous material's surface energy and simulations of high-temperature surface reconstructions (``graphitization''). The presented interatomic potential appears to be promising for realistic and accurate simulations of nanoscale amorphous carbon structures.

Anisotropic high-harmonic generation in bulk crystals

Abstract: High-harmonic generation in a solid turns out to be sensitive to the interatomic bonding — a very useful feature that could enable the all-optical imaging of the interatomic potential. The microscopic valence electron density determines the optical, electronic, structural and thermal properties of materials. However, current techniques for measuring this electron charge density are limited: for example, scanning tunnelling microscopy is confined to investigations at the surface, and electron diffraction requires very thin samples to avoid multiple scattering1. Therefore, an optical method is desirable for measuring the valence charge density of bulk materials. Since the discovery of high-harmonic generation (HHG) in solids2, there has been growing interest in using HHG to probe the electronic structure of solids3,4,5,6,7,8,9,10,11. Here, using single-crystal MgO, we demonstrate that high-harmonic generation in solids is sensitive to interatomic bonding. We find that harmonic efficiency is enhanced (diminished) for semi-classical electron trajectories that connect (avoid) neighbouring atomic sites in the crystal. These results indicate the possibility of using materials’ own electrons for retrieving the interatomic potential and thus the valence electron density, and perhaps even wavefunctions, in an all-optical setting.

Research data supporting "Machine learning based interatomic potential for amorphous carbon"

Abstract: Raw data relevant to the GAP interatomic potential model described in the publication, including output of molecular-dynamics trajectories, DFT reference data, and input files for GAP fitting. Due to the large file sizes, datasets from DFT-based molecular-dynamics simulations ("..._cp2k.tar.gz") and from GAP-based surface simulations ("..._surfaces.tar.gz") are provided as separate archives. The other data, including the GAP fitting input, are included in the main data file.

Transverse discrete breathers in unstrained graphene

Abstract: Discrete breathers (DB) are spatially localized vibrational modes of large amplitude in defect-free nonlinear lattices. The search for DBs in graphene is of high importance, taking into account that this one atom thick layer of carbon is promising for a number of applications. There exist several reports on successful excitation of DBs in graphene, based on molecular dynamics and ab initio simulations. In a recent work by Hizhnyakov with co-authors the possibility to excite a DB with atoms oscillating normal to the graphene sheet has been reported. In the present study we use a systematic approach for finding initial conditions to excite transverse DBs in graphene. The approach is based on the analysis of the frequency-amplitude dependence for a delocalized, short-wavelength vibrational mode. This mode is a symmetry-dictated exact solution to the dynamic equations of the atomic motion, regardless the mode amplitude and regardless the type of interatomic potentials used in the simulations. It is demonstrated that if the AIREBO potential is used, the mode frequency increases with the amplitude bifurcating from the upper edge of the phonon spectrum for out-of-plane phonons. Then a bell-shaped function is superimposed on this delocalized mode to obtain a spatially localized vibrational mode, i.e., a DB. Placing the center of the bell-shaped function at different positions with respect to the lattice sites, three different DBs are found. Typically, the degree of spatial localization of DBs increases with the DB amplitude, but the transverse DBs in graphene reported here demonstrate the opposite trend. The results are compared to those obtained with the use of the Savin interatomic potential and no transverse DBs are found in this case. The results of this study contribute to a better understanding of the nonlinear dynamics of graphene and they call for the ab initio simulations to verify which of the two potentials used in this study is more precise.

Achieving DFT accuracy with a machine-learning interatomic potential: thermomechanics and defects in bcc ferromagnetic iron

Abstract: We show that the Gaussian Approximation Potential machine learning framework can describe complex magnetic potential energy surfaces, taking ferromagnetic iron as a paradigmatic challenging case. The training database includes total energies, forces, and stresses obtained from density-functional theory in the generalized-gradient approximation, and comprises approximately 150,000 local atomic environments, ranging from pristine and defected bulk configurations to surfaces and generalized stacking faults with different crystallographic orientations. We find the structural, vibrational and thermodynamic properties of the GAP model to be in excellent agreement with those obtained directly from first-principles electronic-structure calculations. There is good transferability to quantities, such as Peierls energy barriers, which are determined to a large extent by atomic configurations that were not part of the training set. We observe the benefit and the need of using highly converged electronic-structure calculations to sample a target potential energy surface. The end result is a systematically improvable potential that can achieve the same accuracy of density-functional theory calculations, but at a fraction of the computational cost.

Modified embedded-atom interatomic potential for Fe-Ni, Cr-Ni and Fe-Cr-Ni systems

Abstract: A semi-empirical interatomic potential formalism, the second-nearest-neighbor modified embedded-atom method (2NN MEAM), has been applied to obtaining interatomic potentials for the Fe-Ni, Cr-Ni and Fe-Cr-Ni systems using previously developed MEAM potentials of Fe and Ni and a newly revised potential of Cr. The potential parameters were determined by fitting the experimental data on the enthalpy of formation or mixing, lattice parameter and elastic constant. The present potentials generally reproduced the fundamental physical properties of the Fe-Ni and Cr-Ni alloys. The enthalpy of formation or mixing of the disordered phase at finite temperature and the enthalpy of mixing of the liquid phase are reasonable in agreements with experiment data and CALPHAD calculations. The potentials can be combined with already-developed MEAM potentials to describe Fe-Cr-Ni-based multicomponent alloys. Moreover, the average diffusivities in the unary, some binary and ternary alloys were simulated based on present potential. Good agreement is obtained in comparison with experimental data.

State-of-the-art ab initio potential energy curve for the xenon atom pair and related spectroscopic and thermophysical properties

Abstract: A new ab initio interatomic potential energy curve for two ground-state xenon atoms is presented. It is based on supermolecular calculations at the coupled-cluster level with single, double, and perturbative triple excitations [CCSD(T)] employing basis sets up to sextuple-zeta quality, which were developed as part of this work. In addition, corrections were determined for higher coupled-cluster levels up to CCSDTQ as well as for scalar and spin-orbit relativistic effects at the CCSD(T) level. A physically motivated analytical function was fitted to the calculated interaction energies and used to compute the vibrational spectrum of the dimer, the second virial coefficient, and the dilute gas transport properties. The agreement with the best available experimental data for the investigated properties is excellent; the new potential function is superior not only to previous ab initio potentials but also to the most popular empirical ones.

An empirical potential for simulating vacancy clusters in tungsten.

Abstract: We present an empirical interatomic potential for tungsten, particularly well suited for simulations of vacancy-type defects. We compare energies and structures of vacancy clusters generated with the empirical potential with an extensive new database of values computed using density functional theory, and show that the new potential predicts low-energy defect structures and formation energies with high accuracy. A significant difference to other popular embedded-atom empirical potentials for tungsten is the correct prediction of surface energies. Interstitial properties and short-range pairwise behaviour remain similar to the Ackford-Thetford potential on which it is based, making this potential well-suited to simulations of microstructural evolution following irradiation damage cascades. Using atomistic kinetic Monte Carlo simulations, we predict vacancy cluster dissociation in the range 1100-1300 K, the temperature range generally associated with stage IV recovery.

Optimized interatomic potential for silicon and its application to thermal stability of silicene

Abstract: An optimized interatomic potential has been constructed for silicon using a modified Tersoff model The potential reproduces a wide range of properties of Si and improves over existing potentials with respect to point defect structures and energies, surface energies and reconstructions, thermal expansion, melting temperature, and other properties The proposed potential is compared with three other potentials from the literature The potentials demonstrate reasonable agreement with first-principles binding energies of small Si clusters as well as single-layer and bilayer silicenes The four potentials are used to evaluate the thermal stability of free-standing silicenes in the form of nanoribbons, nanoflakes, and nanotubes While single-layer silicene is found to be mechanically stable at zero Kelvin, it is predicted to become unstable and collapse at room temperature By contrast, the bilayer silicene demonstrates a larger bending rigidity and remains stable at and even above room temperature The results suggest that bilayer silicene might exist in a free-standing form at ambient conditions

Impact of vacancies on the thermal conductivity of graphene nanoribbons: A molecular dynamics simulation study

Abstract: Equilibrium molecular dynamics simulation using 2nd generation Reactive Bond Order interatomic potential has been performed to model the thermal transport of nanometer sized zigzag defected graphene nanoribbons (GNRs) containing several types of vacancies. We have investigated the thermal conductivity of defected GNRs as a function of vacancy concentration within a range of 0.5% to 5% and temperature ranging from 300K to 600K, along with a comparative analysis of those for pristine GNRs. We find that, a vacancy concentration of 0.5% leads to over 90% reduction in the thermal conductivity of GNRs. At low defect concentration, the decay rate is faster but ceases gradually at higher defect concentration. With the increasing temperature, thermal conductivity of defected GNRs decreases but shows less variation in comparison with that of pristine GNRs at higher temperatures. Such comprehensive study on several vacancy type defects in GNRs can provide further insight to tune up the thermal transport characterist...

Conceptual and practical bases for the high accuracy of machine learning interatomic potential

Abstract: Machine learning interatomic potentials (MLIPs) based on a large dataset obtained by density functional theory (DFT) calculation have been developed recently. This study gives both conceptual and practical bases for the high accuracy of MLIPs, although MLIPs have been considered to be simply an accurate black-box description of atomic energy. We also construct the most accurate MLIP of the elemental Ti ever reported using a linearized MLIP framework and many angular-dependent descriptors, which also corresponds to a generalization of the modified embedded atom method (MEAM) potential.

Atomistic Simulations of the Crystallization and Aging of GeTe Nanowires

Abstract: Nanowires made of chalcogenide alloys are of interest for use in phase-change nonvolatile memories. For this application, insights into the thermal properties of such nanowires and, in particular, into the crystallization kinetics at the atomic level are crucial. Toward this end, we have performed large-scale atomistic simulations of ultrathin nanowires (9 nm in diameter) of the prototypical phase-change compound GeTe. We made use of an interatomic potential generated by the neural network fitting of a large ab initio database to compute the thermal properties of the nanowires. By melting a portion of a nanowire, we investigated the velocity of recrystallization as a function of temperature. The simulations show that the melting temperature of the nanowire is about 100 K below the melting temperature of the bulk, which yields a reduction by about a factor of 2 of the maximum crystallization speed. Further, analysis of the structural properties of the amorphous phase of the nanowire suggests a possible ori...

A metastable phase of shocked bulk single crystal copper: an atomistic simulation study

Abstract: Structural phase transformation in bulk single crystal Cu in different orientation under shock loading of different intensities has been investigated in this article. Atomistic simulations, such as, classical molecular dynamics using embedded atom method (EAM) interatomic potential and ab-initio based molecular dynamics simulations, have been carried out to demonstrate FCC-to-BCT phase transformation under shock loading of 〈100〉 oriented bulk single crystal copper. Simulated x-ray diffraction patterns have been utilized to confirm the structural phase transformation before shock-induced melting in Cu(100).

Experimental determination of the interatomic potential in LiNbO3 via ultrafast lattice control

Abstract: We present a direct comparison between resonant terahertz (THz) and nonresonant impulsive stimulated Raman scattering (ISRS) excitation of phonon-polaritons in ferroelectric lithium niobate. THz excitation offers advantages of selectively driving only the forward propagating phonon-polariton mode to exceedingly high amplitudes, without complications due to nonlinear processes at the high 800 nm pump fluences used in Raman excitation. At peak-to-peak THz electric field strengths exceeding 1 MV/cm, the ferroelectric lattice is driven into the anharmonic regime, allowing experimental determination of the shape of the potential energy surface along this vibrational coordinate.

Addressing the discrepancy of finding the equilibrium melting point of silicon using molecular dynamics simulations.

Abstract: We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid-liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.

Grain boundaries in bcc-Fe: a density-functional theory and tight-binding study

Abstract: Grain boundaries (GBs) have a significant influence on material properties. In the present paper, we calculate the energies of eleven low-Σ () symmetrical tilt GBs and two twist GBs in ferromagnetic bcc iron using first-principles density functional theory (DFT) calculations. The results demonstrate the importance of a sufficient sampling of initial rigid body translations in all three directions. We show that the relative GB energies can be explained by the miscoordination of atoms at the GB region. While the main features of the studied GB structures were captured by previous empirical interatomic potential calculations, it is shown that the absolute values of GB energies calculated were substantially underestimated. Based on DFT-calculated GB structures and energies, we construct a new d-band orthogonal tight-binding (TB) model for bcc iron. The TB model is validated by its predictive power on all the studied GBs. We apply the TB model to block boundaries in lath martensite and demonstrate that the experimentally observed GB character distribution can be explained from the viewpoint of interface energy.

Comparing EAM Potentials to Model Slip Transfer of Sequential Mixed Character Dislocations Across Two Symmetric Tilt Grain Boundaries in Ni

Abstract: Slip transfer via sequential pile-up dislocations across grain boundaries (GBs) plays an important role in plastic deformation in polycrystalline face-centered cubic (FCC) metals. In this work, large scale concurrent atomistic-continuum (CAC) method simulations are performed to address the slip transfer of mixed character dislocations across GBs in FCC Ni. Two symmetric tilt GBs, a Σ3{111} coherent twin boundary (CTB) and a Σ11{113} symmetric tilt GB (STGB), are investigated using five different fits to the embedded-atom method (EAM) interatomic potential to assess the variability of predicted dislocation-interface reaction. It is shown that for the Σ3 CTB, two of these potentials predict dislocation transmission while the other three predict dislocation absorption. In contrast, all five fits to the EAM potential predict that dislocations are absorbed by the Σ11 STGB. Simulation results are examined in terms of several slip transfer criteria in the literature, highlighting the complexity of dislocation/GB interactions and the significance of multiscale modeling of the slip transfer process.

The atomistic representation of first strain-gradient elastic tensors

Abstract: We derive the atomistic representations of the elastic tensors appearing in the linearized theory of first strain-gradient elasticity for an arbitrary multi-lattice. In addition to the classical second-Piola) stress and elastic moduli tensors, these include the rank-three double-stress tensor, the rank-five tensor of mixed elastic moduli, and the rank-six tensor of strain-gradient elastic moduli. The atomistic representations are closed-form analytical expressions in terms of the first and second derivatives of the interatomic potential with respect to interatomic distances, and dyadic products of relative atomic positions. Moreover, all expressions are local, in the sense that they depend only on the atomic neighborhood of a lattice site. Our results emanate from the condition of energetic equivalence between continuum and atomistic representations of a crystal, when the kinematics of the latter is governed by the Cauchy–Born rule. Using the derived expressions, we prove that the odd-order tensors vanish if the lattice basis admits central-symmetry. The analytical expressions are implemented as a KIM compliant algorithm to compute the strain gradient elastic tensors for various materials. Numerical results are presented to compare representative interatomic potentials used in the literature for cubic crystals, including simple lattices (fcc Al and Cu and bcc Fe and W) and multi-lattices (diamond-cubic Si). We observe that central potentials exhibit generalized Cauchy relations for the rank-six tensor of strain-gradient elastic moduli. In addition, this tensor is found to be indefinite for many potentials. We discuss the relationship between indefiniteness and material stability. Finally, the atomistic representations are specialized to central potentials in simple lattices. These expressions are used with analytical potentials to study the sensitivity of the elastic tensors to the choice of the cutoff radius.

Interatomic potential to study plastic deformation in tungsten-rhenium alloys

Abstract: In this work, an interatomic potential for the W-Re system is fitted and benchmarked against experimental and density functional theory (DFT) data, of which part are generated in this work. Having in mind studies related to the plasticity of W-Re alloys under irradiation, emphasis is put on fitting point-defect properties, elastic constants, and dislocation properties. The developed potential can reproduce the mechanisms responsible for the experimentally observed softening, i.e., decreasing shear moduli, decreasing Peierls barrier, and asymmetric screw dislocation core structure with increasing Re content in W-Re solid solutions. In addition, the potential predicts elastic constants in reasonable agreement with DFT data for the phases forming non-coherent precipitates (σ- and χ-phases) in W-Re alloys. In addition, the mechanical stability of the different experimentally observed phases is verified in the temperature range of interest (700–1500 K). As a conclusion, the presented potential provides an exce...

Development of a Classical Interatomic Potential for MAPbBr3

Abstract: We develop a classical interatomic potential for MAPbBr3. The model belongs to the class of MYP force-fields for hybrid perovskites based on two-body Buckhingam-Coulomb and dispersive terms to describe organic–inorganic interactions and already successfully applied to MAPbI3. The model calibration is based on a simplified procedure able to extend one existing parametrization to a different halide by suitable scaling of selected subgroups of parameters. The main static and dynamical properties of MAPbBr3 are well reproduced by the developed model: the lattice constant, cohesive energy curve, bulk modulus, energy barriers for cation rotations (both static and dynamic), the phase transition temperatures, and structural parameters evolution with temperature. The model also provides a valid relationship between MAPbBr3 and MAPbI3: MAPbBr3 has shorter lattice constant, higher cohesive energy, lower phase transition temperatures, and larger anisotropy in orthorhombic phase. The good comparison also extends to th...

Strength of the repulsive part of the interatomic potential determines fragility in metallic liquids

Abstract: Liquid fragility is a basic characteristic of glasses and there is debate as to what controls it. X-ray scattering of several metallic liquids now suggests that the strength of the repulsive part of the interatomic potential is key.

Second nearest-neighbor modified embedded-atom method interatomic potentials for the Pt-M (M = Al, Co, Cu, Mo, Ni, Ti, V) binary systems

Abstract: Interatomic potentials for Pt-M (M = Al, Co, Cu, Mo, Ni, Ti, V) binary systems have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The parameters of pure Mo have also been newly developed to solve a problem in the previous 2NN MEAM potential in which the sigma and α-Mn structures become more stable than the bcc structure. The potentials reproduce various materials properties of alloys (structural, thermodynamic and order-disorder transition temperature) in reasonable agreements with relevant experimental data and other calculations. The applicability of the developed potentials to atomistic investigations for the shape and atomic configuration of Pt bimetallic nanoparticles is demonstrated.

A force-matching Stillinger-Weber potential for MoS2: Parameterization and Fisher information theory based sensitivity analysis

Abstract: Two-dimensional molybdenum disulfide (MoS2) is a promising material for the next generation of switchable transistors and photodetectors. In order to perform large-scale molecular simulations of the mechanical and thermal behavior of MoS2-based devices, an accurate interatomic potential is required. To this end, we have developed a Stillinger-Weber potential for monolayer MoS2. The potential parameters are optimized to reproduce the geometry (bond lengths and bond angles) of MoS2 in its equilibrium state and to match as closely as possible the forces acting on the atoms along a dynamical trajectory obtained from ab initio molecular dynamics. Verification calculations indicate that the new potential accurately predicts important material properties including the strain dependence of the cohesive energy, the elastic constants, and the linear thermal expansion coefficient. The uncertainty in the potential parameters is determined using a Fisher information theory analysis. It is found that the parameters are...

Thermal transport characterization of hexagonal boron nitride nanoribbons using molecular dynamics simulation

Abstract: Due to similar atomic bonding and electronic structure to graphene, hexagonal boron nitride (h-BN) has broad application prospects such as the design of next generation energy efficient nano-electronic devices. Practical design and efficient performance of these devices based on h-BN nanostructures would require proper thermal characterization of h-BN nanostructures. Hence, in this study we have performed equilibrium molecular dynamics (EMD) simulation using an optimized Tersoff-type interatomic potential to model the thermal transport of nanometer sized zigzag hexagonal boron nitride nanoribbons (h-BNNRs). We have investigated the thermal conductivity of h-BNNRs as a function of temperature, length and width. Thermal conductivity of h-BNNRs shows strong temperature dependence. With increasing width, thermal conductivity increases while an opposite pattern is observed with the increase in length. Our study on h-BNNRs shows considerably lower thermal conductivity compared to GNRs. To elucidate these aspect...

Study of NaF–AlF3 Melts by Coupling Molecular Dynamics, Density Functional Theory, and NMR Measurements

Abstract: Improvement of the industrial electrolytic process for aluminum production necessitates a thorough understanding of the underlying ionic structure of the electrolyte, which mainly comprises NaF and AlF3 at around 965 °C. The chemical and physical properties of this melt strongly depend on the aluminum speciation, which requires a multipronged approach in order to clarify its properties. Here we parametrize a new polarizable ion model (PIM) interatomic potential for the molten NaF–AlF3 system, which is used to study the liquid properties up to 50 mol % of AlF3 at high temperatures. The potential parameters are obtained by force fitting to density functional theory (DFT) reference data. Molecular dynamics (MD) simulations are combined with further DFT calculations to determine NMR chemical shifts for 27Al, 23Na, and 19F. An excellent agreement is obtained with experimental values. This enables the study of the dynamic properties of the melts such as viscosity, electrical conductivity, and self-diffusion coe...