Charge-optimized Many-body Research Articles

Fitting the charged-optimized many-body potential for the Al-O-Se-Zn system

A charge-optimized many-body (COMB3) potential has been developed for the Al-O-Se-Zn system using first-principles data. This work updates COMB3 for zinc and extends it to include selenium as well as other pairwise and three-body interactions that were not developed previously. We tested the empirical potential for known crystalline phases and find reasonable agreement between the calculated structural, elastic, and vibrational properties and experimental data. We use the anisotropic coefficients of the thermal expansion method to calculate the temperature-dependent properties of a few selected materials. We then compare the total energy of the system and its first- and second-order positional derivatives, as well as the second-order strain derivative with respect to the atomic potential energy surface to thermal properties obtained from molecular dynamics simulations using the fitted COMB3 potential. The results of our temperature-dependent calculations show reasonable agreement with previous work for temperatures below the system’s melting point. With this fitting, we could utilize a versatile, charge-dependent empirical potential to model the interface between ZnSe and Al2O3 by molecular dynamics simulations.

Charge-optimized many-body interaction potential for AlN revisited to explore plasma–surface interactions

Plasma–surface interactions during AlN thin film sputter deposition could be studied by means of reactive molecular dynamics (RMD) methods. This requires an interaction potential that describes all species as well as wall interactions (e.g., particle emission, damage formation) appropriately. However, previous works focused on the establishment of AlN bulk potentials. Although for the third-generation charge-optimized many-body (COMB3) potential at least a single reference surface was taken into account, surface interactions are subject to limited reliability only. The demand for a revised COMB3 AlN potential is met in two steps: First, the Ziegler–Biersack–Littmark potential is tapered and the variable charge model QTE^{+} is implemented to account for high-energy collisions and distant charge transport, respectively. Second, the underlying parameterization is reworked by applying a self-adaptive evolution strategy implemented in the GARFfield software. Four wurtzite, three zinc blende and three rock salt surfaces are considered. An example study on the ion bombardment induced particle emission and point defect formation reveals that the revised COMB3 AlN potential is appropriate for the accurate investigation of plasma–surface interactions by means of RMD simulations.

Atomistic Investigation of the Titanium Carbide MXenes under Impact Loading

2D Titanium carbide MXenes with a structural formula recognized as Tin+1Cn have attracted attention from both the academic and industry fields due to their intriguing mechanical properties and appealing potential in a variety of areas such as nano-electronic circuits/devices, bio sensors, energy storage and reinforcing material for composites. Based on mutli-body comb3 (third-generation Charge-Optimized Many-Body) potential, this work investigated the impact resistance of monolayer Tin+1Cn nanosheets (namely, Ti2C Ti3C2 and Ti4C3) under hypervelocity up to 7 km/s. The deformation behavior and the impact resist mechanisms of Tin+1Cn nanosheets were assessed. Penetration energy is found to positively correlate with the number of titanium atom layer (n). However, in tracking atomic Von Mises stress distribution, Ti2C exhibits the most significant elastic wave propagation velocity among the examined nanosheets, suggesting the highest energy delocalization rate and stronger energy dissipation via deformation prior to bond break. Consistently, Ti2C presents superior specific penetration energy due its Young’s-modulus-to-density ratio, followed by Ti3C2 and Ti4C3, suggesting an inverse correlation between the titanium atom layer number and specific penetration energy. This study provides a fundamental understanding of the deformation and penetration mechanisms of titanium carbide MXene nanosheets under impact, which could be beneficial to facilitating their emerging impact protection applications.

A charge optimized many-body potential for iron/iron-fluoride systems.

A classical interatomic potential for iron/iron-fluoride systems is developed in the framework of the charge optimized many-body (COMB) potential. This interatomic potential takes into consideration the effects of charge transfer and many-body interactions depending on the chemical environment. The potential is fitted to a training set composed of both experimental and ab initio results of the cohesive energies of several Fe and FeF2 crystal phases, the two fluorine molecules F2 and the F2-1 dissociation energy curve, the Fe and FeF2 lattice parameters of the ground state crystalline phase, and the elastic constants of the body centered cubic Fe structure. The potential is tested in an NVT ensemble for different initial structural configurations as the crystal ground state phases, F2 molecules, iron clusters, and iron nanospheres. In particular, we model the FeF2/Fe bilayer and multilayer interfaces, as well as a system of square FeF2 nanowires immersed in an iron solid. It has been shown that there exists a reordering of the atomic positions for F and Fe atoms at the interface zone; this rearrangement leads to an increase in the charge transfer among the atoms that make the interface and put forward a possible mechanism of the exchange bias origin based on asymmetric electric charge transfer in the different spin channels.

A universal COMB potential for the whole composition range of the uranium[sbnd]oxygen system

An empirical molecular dynamics potential in the Charge-Optimized Many-Body (COMB) formalism that covers the whole uranium-oxygen composition range has been developed. Extended from a previous potential for uranium metal, this universal UO potential is able to model more than 20 phases of uranium oxides. The potential's flexibility, accuracy and transferability have been fully verified by rigorous testing and comparison with ab-initio calculations and experimental measurements. It is shown to be one of the most versatile and high-quality UO2 potentials, the first potential for U4O9, U3O7 and UO3, and the first usable U3O8 potential. Many important properties of major oxides in the UO phase diagram have been calculated and critically reviewed, including the cohesive energy, formation/reaction energies, lattice parameters, elastic constants, bulk/shear moduli, and energies for non-stoichiometric point defects and stoichiometric defects pairs. Due to its special design and parameterization process, this UO potential is shown to outperform all other existing ones by either obtaining higher accuracy for many of these quantities, or exclusively being able to calculate some of them. The successfully development of this potential provides a useful, reliable and convenient tool for molecular dynamics simulations that are previously impossible or unreliable to do for many materials in the UO system. Correction for several published oxide structures is also included in the appendix.

Charge Optimized Many Body (COMB) potentials for simulation of nuclear fuel and clad

We briefly outline the Charge Optimized Many Body (COMB) potential formalism, which enables the molecular dynamics simulation of complex materials structures in which multiple types of bonding (metallic, covalent, ionic and secondary bonding) coexist. We illustrate its capabilities to address critical issues in the area of nuclear fuel. In particular, we look at U, UO2 and the process of oxidation of U. Further, we characterize the mechanical behavior of Zr, representing LWR clad, and explore the effects of oxidation and hydridation on the mechanical response and briefly illustrate the capabilities of COMB simulations of corrosion. Finally, we briefly assess the materials fidelity of the COMB approach by examining the COMB description for the Zr-H system.

Effect of annealing on structural changes and oxygen diffusion in amorphous HfO2 using classical molecular dynamics

Non-volatile memory is a promising alternative to present memory technologies. Oxygen vacancy diffusion has been widely accepted as one of the reasons for the resistive switching mechanism of transition-metal-oxide based resistive random access memory. In this study, molecular dynamics simulation is applied to investigate the diffusion coefficient and activation energy of oxygen in amorphous hafnia. Two sets of empirical potential, Charge-Optimized Many-Body (COMB) and Morse-BKS (MBKS), were considered to investigate the structural and diffusion properties at different temperatures. COMB predicts the activation energy of 0.53 eV for the temperature range of 1000–2000 K, while MBKS predicts 2.2 eV at high temperature (1600–2000 K) and 0.36 eV at low temperature (1000–1600 K). Structural changes and appearance of nano-crystalline phases with increasing temperature might affect the activation energy of oxygen diffusion predicted by MBKS, which is evident from the change in coordination number distribution and radial distribution function. None of the potentials make predictions that are fully consistent with density functional theory simulations of both the structure and diffusion properties of HfO2. This suggests the necessity of developing a better multi-body potential that considers charge exchange.

Applied Potentials in Variable-Charge Reactive Force Fields for Electrochemical Systems.

An atomic description of water dynamics and electrochemical properties at electrode-electrolyte interfaces is presented using molecular dynamics with the third generation of the charge-optimized many-body (COMB3) potential framework. Externally applied potentials in electrochemical applications were simulated by offsetting electronegativity on electrode atoms. This approach is incorporated into the variable charge scheme within COMB3 and is used to investigate electrochemical systems consisting of two Cu electrodes and a water electrolyte with varying concentrations of hydroxyls (OH-) and protons (H+). The interactions between the electronegativity offset method and the charge equilibration method in a variable charge scheme are analyzed. In addition, a charge equilibration method for electrochemical applications is proposed, where the externally applied potentials are treated by the electronegativity offset on the electrodes thus enforcing charge neutrality on the electrolyte. This method is able to qualitatively capture the relevant electrochemistry and predict consistently correct voltages with precalibration.

Phonon optimized interatomic potential for aluminum

We address the problem of generating a phonon optimized interatomic potential (POP) for aluminum. The POP methodology, which has already been shown to work for semiconductors such as silicon and germanium, uses an evolutionary strategy based on a genetic algorithm (GA) to optimize the free parameters in an empirical interatomic potential (EIP). For aluminum, we used the Vashishta functional form. The training data set was generated ab initio, consisting of forces, energy vs. volume, stresses, and harmonic and cubic force constants obtained from density functional theory (DFT) calculations. Existing potentials for aluminum, such as the embedded atom method (EAM) and charge-optimized many-body (COMB3) potential, show larger errors when the EIP forces are compared with those predicted by DFT, and thus they are not particularly well suited for reproducing phonon properties. Using a comprehensive Vashishta functional form, which involves short and long-ranged interactions, as well as three-body terms, we were able to better capture interactions that reproduce phonon properties accurately. Furthermore, the Vashishta potential is flexible enough to be extended to Al2O3 and the interface between Al-Al2O3, which is technologically important for combustion of solid Al nano powders. The POP developed here is tested for accuracy by comparing phonon thermal conductivity accumulation plots, density of states, and dispersion relations with DFT results. It is shown to perform well in molecular dynamics (MD) simulations as well, where the phonon thermal conductivity is calculated via the Green-Kubo relation. The results are within 10% of the values obtained by solving the Boltzmann transport equation (BTE), employing Fermi’s Golden Rule to predict the phonon-phonon relaxation times.

A third-generation charge optimized many body (COMB3) potential for nitrogen-containing organic molecules

This work presents a new empirical, variable charge potential for NCOH systems in the third-generation charge-optimized many-body (COMB3) potential framework. The potential parameters were determined by fitting each binary system separately to the experimental enthalpy of formation and geometries for experimentally important molecules. The resulting parameter set reproduces the energetics and geometries of many nitrogen-based organic molecules with a similar average error as other reactive potentials for the same systems.

Charge optimized many body (COMB) potentials for Pt and Au

Interatomic potentials for Pt and Au are developed within the third generation charge optimized many-body (COMB3) formalism. The potentials are capable of reproducing phase order, lattice constants, and elastic constants of Pt and Au systems as experimentally measured or calculated by density functional theory. We also fit defect formation energies, surface energies and stacking fault energies for Pt and Au metals. The resulting potentials are used to map a 2D contour of the gamma surface and simulate the tensile test of 16-grain polycrystalline Pt and Au structures at 300 K. The stress–strain behaviour is investigated and the primary slip systems {1 1 1}〈1 0〉 are identified. In addition, we perform high temperature (1800 K for Au and 2300 K for Pt) molecular dynamics simulations of 30 nm Pt and Au truncated octahedron nanoparticles and examine morphological changes of each particle. We further calculate the activation energy barrier for surface diffusion during simulations of several nanoseconds and report energies of eV for Pt and eV for Au. This initial parameterization and application of the Pt and Au potentials demonstrates a starting point for the extension of these potentials to multicomponent systems within the COMB3 framework.

Adhesion properties of Cu(111)/α-quartz (0001) interfaces: A molecular dynamics study

The fundamental properties of Cu/SiO2 interface are worth studying because they impact the quality and performance of silicon-based microelectronics and related devices. Using the charge-optimized many-body (COMB) potential in this study, we present a molecular dynamics simulation study of the structural, adhesive and electronic properties of Cu(111)/α-quartz SiO2 (0001) interfaces with two different crystalline orientations and various terminations by double-oxygens (OO), single-oxygen(O) and silicon(Si). For the equilibrated interfaces, the largest adhesion energies correspond to the oxygen richest OO-terminated interface in which the oxidation level of Cu is highest due to the largest charge transfer across the interface. In particular, we also investigate the properties of a series of nonequilibrated OO-, O- and Si-terminated interfaces that are created from their equilibrated counterparts by introducing vacancies of different numbers and different types. It is found that the adhesion energies of interfaces mostly decrease upon vacancy introductions only except for Si vacancies added in the Si-terminated interface. For all nonequilibrated interfaces of different terminations, we found a linear correlation between adhesive energy and area average excess charge transfer in Cu.

Nanoindentation of ZrO2 and ZrO2/Zr systems by molecular dynamics simulation

The deformation behaviors of cubic zirconia and a cubic zirconia thin film on top of an hcp zirconium substrate are investigated using molecular dynamics nanoindentation simulation. Interatomic interactions are described by the previously developed Charge Optimized Many Body (COMB) potential for the Zr-ZrO2-O2 system. The load-displacement curves, deformation processes and hardnesses of zirconia and the zirconia/zirconium systems are characterized. In addition, by comparing with a previous nanoindentation simulation on zirconium, the effects of the zirconia layer on top on the mechanical properties of the zirconium substrate are determined.

Effect of Surface Chemistry on Water Interaction with Cu(111).

The interfacial dynamics of water in contact with bare, oxidized, and hydroxylated copper surfaces are examined using classical molecular dynamics (MD) simulations. A third-generation charge-optimized many-body (COMB3) potential is used in the MD simulations to investigate the adsorption of water molecules on Cu(111), and the results are compared to the findings of density functional theory (DFT) calculations. The adsorption energies and structures predicted by COMB3 are generally consistent with those determined with DFT. The COMB3 potential is then used to investigate the wetting behavior of water nanodroplets on Cu(111) at 20, 130, and 300 K. At room temperature, the simulations predict that the spreading rate of the base radius, R0, of a water droplet with a diameter of about 1.5 nm exhibits a spreading rate of R0 ≈ t(0.16) and a final base radius of 3.5 nm. At 20 and 130 K, water droplets are predicted to retain their structure after adsorption on Cu(111) and to undergo minimal spreading in agreement with scanning tunneling microscopy data. When the same water droplet encounters a reconstructed, oxidized Cu(111) surface, the classical MD simulations predict wetting with a spreading rate of R ≈ t(0.14) and a final base radius of 3.0 nm. Similarly, our MD simulations predict a spreading rate of R ≈ t(0.14) and a final base radius of 2.5 nm when water encounters OH-covered Cu(111). These results indicate that oxidation and hydroxylation cause a reduction in the degree of spreading and final base radius that is directly associated with a decreased spreading rate for water nanodroplets on copper.

Homogeneous hydride formation path in α-Zr: Molecular dynamics simulations with the charge-optimized many-body potential

A path for homogeneous γ hydride formation in hcp α-Zr, from solid solution to the ζ and then the γ hydride, was demonstrated using molecular static calculations and molecular dynamic simulations with the charge-optimized many-body (COMB) potential. Hydrogen has limited solubility in α-Zr. Once the solubility limit is exceeded, the stability of solid solution gives way to that of coherent hydride phases such as the ζ hydride by planar precipitation of hydrogen. At finite temperatures, the ζ hydride goes through a partial hcp-fcc transformation via 13〈11¯00〉 slip on the basal plane, and transforms into a mixture of γ hydride and α-Zr. In the ζ hydride, slip on the basal plane is favored thermodynamically with negligible barrier, and is therefore feasible at finite temperatures without mechanical loading. The transformation process involves slips of three equivalent shear partials, in contrast to that proposed in the literature where only a single shear partial was involved. The adoption of multiple slip partials minimizes the macroscopic shape change of embedded hydride clusters and the shear strain accumulation in the matrix, and thus reduces the overall barrier needed for homogeneous γ hydride formation. This formation path requires finite temperatures for hydrogen diffusion without mechanical loading. Therefore, it should be effective at the cladding operating conditions.

Lattice expansion by intrinsic defects in uranium by molecular dynamics simulation

A re-formulated and re-parameterized interatomic potential for uranium metal in the Charge-Optimized Many-Body (COMB) formalism is presented. Most physical properties of the orthorhombic α and bcc γ phases are accurately reproduced. In particular, this potential can reproduce the negative thermal expansion of the b axis in α-U while keeping this phase as the most stable phase at low temperatures, in accord with experiment. Most of the volume expansion in α-U by intrinsic defects is shown to come from the b axis, due to the formation of prismatic loops normal to this direction. Glide dislocation loops forming stacking faults are also observed. Structures of both loop types are analyzed. An expansion simulation is conducted and the results are verified by using the Norgett-Robinson-Torrens model. Rather than forming extended defect structures as in α-U, the γ phase forms only isolated defects and thus results in a much smaller and isotropic expansion.

Bond-order reactive force fields for molecular dynamics simulations of crystalline silica

This paper investigates the applicability of the bond-order, variable-charge (BOVC) force fields of the Charge-Optimized Many-Body (COMB10), ReaxFFSiOH2O, and ReaxFFSiOGSI, for molecular dynamics (MD) simulations of crystalline SiO2. The calculated lattice constants and densities of the four SiO2 polymorphs, quartz, cristobalite, coesite, and stishovite, are compared to experimental values. Additionally, the calculated pair distribution and bond-angle distribution functions and the α–β transition for quartz, the most stable low-energy polymorph, are compared to experimental results. The simulations with the COMB10 force field accurately predict the properties of the SiO2 polymorphs, except the α-cristobalite, and the quartz α–β transition. The results with ReaxFFSiOH2O and ReaxFFSiOGSI accurately predict the properties of the SiO2 polymorphs, except the stishovite, but those with ReaxFFSiOH2O inaccurately predict the quartz α–β transition.

Charge optimized many-body (COMB) potential for dynamical simulation of Ni–Al phases

An interatomic potential for the Ni–Al system is presented within the third-generation charge optimized many-body (COMB3) formalism. The potential has been optimized for Ni3Al, or the γ′ phase in Ni-based superalloys. The formation energies predicted for other Ni–Al phases are in reasonable agreement with first-principles results. The potential further predicts good mechanical properties for Ni3Al, which includes the values of the complex stacking fault (CSF) and the anti-phase boundary (APB) energies for the (1 1 1) and (1 0 0) planes. It is also used to investigate dislocation propagation across the Ni3Al (1 1 0)–Ni (1 1 0) interface, and the results are consistent with simulation results reported in the literature. The potential is further used in combination with a recent COMB3 potential for Al2O3 to investigate the Ni3Al (1 1 1)–Al2O3 (0 0 01) interface, which has not been modeled previously at the classical atomistic level due to the lack of a reactive potential to describe both Ni3Al and Al2O3 as well as interactions between them. The calculated work of adhesion for this interface is predicted to be 1.85 J m−2, which is in agreement with available experimental data. The predicted interlayer distance is further consistent with the available first-principles results for Ni (1 1 1)–Al2O3 (0 0 0 1).

Charge optimized many-body (COMB) potential for Al2O3 materials, interfaces, and nanostructures

This work presents the development and applications of a new empirical, variable charge potential for Al2O3 systems within the charge optimized many-body (COMB) potential framework. The potential can describe the fundamental physical properties of Al2O3, including cohesive energy, elastic constants, defect formation energies, surface energies and phonon properties of α-Al2O3 comparable to that obtained from experiments and first-principles calculations. The potential is further employed in classical molecular dynamics (MD) simulations to validate and predict the properties of the Al (1 1 1)–Al2O3 (0 0 0 1) interface, tensile properties of Al nanowires, Al2O3 nanowires, Al2O3-covered Al nanowires, and defective Al2O3 nanowires. The results demonstrate that the potential is well-suited to model heterogeneous material systems involving Al and Al2O3. Most importantly, the parameters can be seamlessly coupled with COMB3 parameters for other materials to enable MD simulations of a wide range of heterogeneous material systems.

A charge optimized many-body (comb) potential for titanium and titania

This work proposes an empirical, variable charge potential for Ti and TiO2 systems based on the charge-optimized many-body (COMB) potential framework. The parameters of the potential function are fit to the structural and mechanical properties of the Ti hcp phase, the TiO2 rutile phase, and the energetics of polymorphs of both Ti and TiO2. The relative stabilities of TiO2 rutile surfaces are predicted and compared to the results of density functional theory (DFT) and empirical potential calculations. The transferability of the developed potential is demonstrated by determining the adsorption energy of Cu clusters of various sizes on the rutile TiO2(1 1 0) surface using molecular dynamics simulations. The results indicate that the adsorption energy is dependent on the number of Cu–Cu bonds and Cu–O bonds formed at the Cu/TiO2 interface. The adsorption energies of Cu clusters on the reduced and oxidized TiO2(1 1 0) surfaces are also investigated, and the COMB potential predicts enhanced bonding between Cu clusters and the oxidized surface, which is consistent with both experimental observations and the results of DFT calculations for other transition metals (Au and Ag) on this oxidized surface.