Many-body Potential Research Articles

The phase diagram of carbon dioxide from correlation functions and a many-body potential.

The phase stability and equilibria of carbon dioxide are investigated from 125-325K and 1-10 000 atm using extensive molecular dynamics (MD) simulations and the Two-Phase Thermodynamics (2PT) method. We devise a direct approach for calculating phase diagrams, in general, by considering the separate chemical potentials of the isolated phase at specific points on the P-T diagram. The unique ability of 2PT to accurately and efficiently approximate the entropy and Gibbs energy of liquids allows for assignment of phase boundaries from relatively short (∼100ps) MD simulations. We validate our approach by calculating the critical properties of the flexible elementary physical model 2, showing good agreement with previous results. We show, however, that the incorrect description of the short-range Pauli force and the lack of molecular charge polarization lead to deviations from experiments at high pressures. We, thus, develop a many-body, fluctuating charge model for CO2, termed CO2-Fq, from high level quantum mechanics (QM) calculations that accurately capture the condensed phase vibrational properties of the solid (including the Fermi resonance at 1378cm-1) as well as the diffusional properties of the liquid, leading to overall excellent agreement with experiments over the entire phase diagram. This work provides an efficient computational approach for determining phase diagrams of arbitrary systems and underscores the critical role of QM charge reorganization physics in molecular phase stability.

A new interatomic potential describing Fe-H and H-H interactions in bcc iron

We present a new many-body interatomic potential for H in body-centered cubic (bcc) Fe. The potential is developed based on extensive energetics and atomic configurations of an H atom and H-H interactions in Fe from density functional theory calculations. In detail, the potential is parameterized by fitting not only to a single H atom in the perfect bcc Fe lattice and to the properties of H trap binding to a vacancy and surfaces as being done by previous studies, but also to multiple H trapping to a vacancy and H-H interaction in Fe lattice. With such a fitting strategy, the developed potential outperforms existing potentials in its ability not only describing the behaviors of a single H atom in Fe, but also capturing the features of H-H interaction reliably, which is of key importance in revealing H behaviors in local H accumulation around dislocation cores, grain boundaries and crack tips.

Highly efficient parallel grand canonical simulations of interstitial-driven diffusion-deformation processes

Absorption of interstitial alloying elements like H, O, C, and N in metals and their continuous relocation and interactions with various microstructural features such as vacancies, dislocations, and grain boundaries have crucial influences on metals’ properties. However, besides limitations in experimental tools in capturing these mechanisms, the inefficiency of numerical tools also inhibits modeling efforts. Here, we present an efficient framework to perform hybrid grand canonical Monte Carlo and molecular dynamics simulations that allow for parallel insertion/deletion of Monte Carlo moves. A new methodology for calculation of the energy difference at trial moves that can be applied to many-body potentials as well as pair ones is a primary feature of our implementation. We study H diffusion in Fe (ferrite phase) and Ni polycrystalline samples to demonstrate the efficiency and scalability of the algorithm and its application. The computational cost of using our framework for half a million atoms is a factor of 250 less than the cost of using existing libraries.

Vapor-liquid equilibrium of water with the MB-pol many-body potential.

Among the many existing molecular models of water, the MB-pol many-body potential has emerged as a remarkably accurate model, capable of reproducing thermodynamic, structural, and dynamic properties across water's solid, liquid, and vapor phases. In this work, we assessed the performance of MB-pol with respect to an important set of properties related to vapor-liquid coexistence and interfacial behavior. Through direct coexistence classical molecular dynamics simulations at temperatures of 400K < T < 600K, we calculated properties such as equilibrium coexistence densities, vapor-liquid interfacial tension, vapor pressure, and enthalpy of vaporization and compared the MB-pol results to experimental data. We also compared rigid vs fully flexible variants of the MB-pol model and evaluated system size effects for the properties studied. We found that the MB-pol model predictions are in good agreement with experimental data, even for temperatures approaching the vapor-liquid critical point; this agreement was largely insensitive to system sizes or the rigid vs flexible treatment of the intramolecular degrees of freedom. These results attest to the chemical accuracy of MB-pol and its high degree of transferability, thus enabling MB-pol's application across a large swath of water's phase diagram.

Highly Accurate Many-Body Potentials for Simulations of N2O5 in Water: Benchmarks, Development, and Validation.

Dinitrogen pentoxide (N2O5) is an important intermediate in the atmospheric chemistry of nitrogen oxides. Although there has been much research, the processes that govern the physical interactions between N2O5 and water are still not fully understood at a molecular level. Gaining a quantitative insight from computer simulations requires going beyond the accuracy of classical force fields while accessing length scales and time scales that are out of reach for high-level quantum-chemical approaches. To this end, we present the development of MB-nrg many-body potential energy functions for nonreactive simulations of N2O5 in water. This MB-nrg model is based on electronic structure calculations at the coupled cluster level of theory and is compatible with the successful MB-pol model for water. It provides a physically correct description of long-range many-body interactions in combination with an explicit representation of up to three-body short-range interactions in terms of multidimensional permutationally invariant polynomials. In order to further investigate the importance of the underlying interactions in the model, a TTM-nrg model was also devised. TTM-nrg is a more simplistic representation that contains only two-body short-range interactions represented through Born-Mayer functions. In this work, an active learning approach was employed to efficiently build representative training sets of monomer, dimer, and trimer structures, and benchmarks are presented to determine the accuracy of our new models in comparison to a range of density functional theory methods. By assessing the binding curves, distortion energies of N2O5, and interaction energies in clusters of N2O5 and water, we evaluate the importance of two-body and three-body short-range potentials. The results demonstrate that our MB-nrg model has high accuracy with respect to the coupled cluster reference, outperforms current density functional theory models, and thus enables highly accurate simulations of N2O5 in aqueous environments.

Spectral decomposition of thermal conductivity: Comparing velocity decomposition methods in homogeneous molecular dynamics simulations

The design of new applications, especially those based on heterogeneous integration, must rely on detailed knowledge of material properties, such as thermal conductivity (TC). To this end, multiple methods have been developed to study TC as a function of vibrational frequency. Here, we compare three spectral TC methods based on velocity decomposition in homogenous molecular dynamics simulations: Green-Kubo modal analysis (GKMA), the spectral heat current (SHC) method, and a method we propose called homogeneous nonequilibrium modal analysis (HNEMA). First, we derive a convenient per-atom virial expression for systems described by general many-body potentials, enabling compact representations of the heat current, each velocity decomposition method, and other related quantities. Next, we evaluate each method by calculating the spectral TC for carbon nanotubes, graphene, and silicon. We show that each method qualitatively agrees except at optical phonon frequencies, where a combination of mismatched eigenvectors and a large density of states produces artificial TC peaks for modal analysis methods. Our calculations also show that the HNEMA and SHC methods converge much faster than the GKMA method, with the SHC method being the most computationally efficient. Finally, we demonstrate that our single-GPU modal analysis implementation in GPUMD (Graphics Processing Units Molecular Dynamics) is over 1000 times faster than the existing LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) implementation on one CPU.

Investigation of structural and energetic behavior of 55-atom Pt–Ag–Au ternary nanoalloys

A systematic theoretical investigation of structural and energetic behaviors of 55-atom Pt–Ag–Au ternary nanoalloys has been performed in two different composition systems. We have performed Gupta and Density Functional Theory (DFT) approaches on chosen systems. The Basin-Hopping algorithm is used for structural optimizations of PtnAg[Formula: see text]Au[Formula: see text] ([Formula: see text]–13) and PtnAu[Formula: see text]Ag[Formula: see text] ([Formula: see text]–13) ternary nanoalloys with Gupta many-body potential to model interatomic interactions. Local optimization results show that while the tendency of Au atoms to be located varies according to the composition system, the tendency of Pt and Ag atoms to be located does not change in both. For all compositions of Pt–Ag–Au nanoalloys, the structures with the best chemical ordering were then reoptimized by DFT relaxations and the mixing energies of the Gupta and DFT levels were compared. Our mixing energy analysis showed that PtnAg[Formula: see text]Au[Formula: see text] ([Formula: see text]–13) nanoalloys are not energetically suitable for mixing at both Gupta and DFT level. Also, mixing energy variations of PtnAu[Formula: see text]Ag[Formula: see text] ([Formula: see text]–13) nanoalloys obtained at Gupta level does not agree with the one obtained at DFT level. In addition, it has been found that the minimization energy changes when an atom in the central site is exchanging by an atom in the second shell and surface.

Convergence of stochastic-extended Lagrangian molecular dynamics method for polarizable force field simulation

Extended Lagrangian molecular dynamics (XLMD) is a general method for performing molecular dynamics simulations using quantum and classical many-body potentials. Recently several new XLMD schemes have been proposed and tested on several classes of many-body polarization models such as induced dipoles or Drude charges, by creating an auxiliary set of these same degrees of freedom that are reversibly integrated through time. This gives rise to a singularly perturbed Hamiltonian system that provides a good approximation to the time evolution of the real mutual polarization field. To further improve upon the accuracy of the XLMD dynamics in the context of classical polarizable force field simulation, and to potentially extend it to other many-body potentials, we introduce a stochastic modification which leads to a set of singularly perturbed Langevin equations with degenerate noise. We prove that the resulting Stochastic-XLMD converges to the accurate dynamics, and the convergence rate is both sharp and is independent of the accuracy of the initial polarization field. We carefully study the scaling of the damping factor and numerical noise for efficient numerical simulation for Stochastic-XLMD, and we demonstrate the effectiveness of the method for water molecules described by a polarizable force field.

Hbox {Ag}_{m} hbox {Rh}_n clusters with m+nle 55

Using a combination of genetic algorithms for the unbiased structure optimization and a Gupta many-body potential for the calculation of the energetic properties of a given structure, we determine the putative total-energy minima for all hbox {Ag}_{m} hbox {Rh}_n clusters with a total number of atoms m+n up to 55. Subsequently, we use various descriptors to analyze the obtained structural and energetic properties. With the help of a similarity function, we show that the pure Ag and Rh clusters are structurally similar for sizes up to around 20 atoms. The same approach gives that the mixed clusters tend to possess a larger structural similarity with the pure Rh clusters than with the pure Ag clusters. However, for clusters with msimeq nge 25, other structures dominate. The effective coordination numbers for the Ag and Rh atoms as well as the radial distributions of those atoms indicate that there is a tendency towards segregation with Rh atoms forming an inner part and the Ag atoms forming a shell. Only few clusters, all with a fairly large total number of atoms, are found to be particularly stable.

Investigation of the chemical ordering for PdnPt(32−n)Rh6 nanoalloys in TO structure

Using basin-hopping algorithm within the quantum corrected Sutton-Chen (Q-SC) many-body potential, a systematic investigation has been performed for the best chemical ordering structures of 38-atom trimetallic PdnPt[Formula: see text]Rh6 nanoalloys with truncated octahedral (TO) geometry. The atomic mixing degrees of Pd, Pt and Rh atoms were investigated by using order parameter ([Formula: see text]). The results show that demixing is energetically favorable for the investigated structures, where the number of Rh atoms fixed at 6. The only compositions with the number of hetero bonds lower than the homo bonds are Pt[Formula: see text]Rh6, Pd1Pt[Formula: see text]Rh6, Pd[Formula: see text]Pt[Formula: see text]Rh6 and Pd[Formula: see text]Rh6 which make segregation energetically favorable. The surface segregation of Pd atoms is explained by the lower cohesive energy of Pd atoms than Pt and Rh. The segregation of Pd atoms to the surface is also associated with the smaller surface energies compared with Pt and Rh atoms. Due to the comparable cohesive energies of Rh and Pt atoms, a computation occurs for filling the octahedral core between two kinds.

Heating rate effects for the melting transition of Pt–Ag–Au nanoalloys

The classical molecular dynamics simulations in canonical NVT ensemble conditions are used to investigate the melting transition in different heating rates of Pt–Ag–Au ternary nanoalloys. In order to obtain the initial configurations used in the molecular dynamics simulations, optimizing the chemical ordering of Pt13Ag n Au42 – n (n = 0–42) ternary nanoalloys was performed using the Basin–Hopping algorithm which would not allow changes in the icosahedron structure. The Gupta many-body potential was used to model interatomic interactions in both molecular dynamics simulations and optimization simulations. The melting transitions of selected Pt–Ag–Au nanoalloys were explored using caloric curves and Lindemann parameters. There have been two identified types of melting mechanisms, one includes sudden jump behavior in the caloric curve and the other is an isomerization while melting transition. The temperature range in which the isomerization takes place depends on the heating rate value.

Modeling of supersonic crowdion clusters in FCC lattice: Effect of the interatomic potential

Among a wide variety of point defects, crowdions can be distinguished by their high energy of formation and relatively low migration barriers, which makes them an important agent of mass transfer in lattices subjected to severe plastic deformation, irradiation, etc. It was previously shown that complexes and clusters of crowdions are even more mobile than single interstitials, which opened new mechanisms for the transfer of energy and mass in materials under intense external impacts. One of the most popular and convenient methods for analyzing crowdions is molecular dynamics, where the results can strongly depend on the interatomic potential used in the study. In this work, we compare the characteristics of a crowdion in an fcc lattice obtained using two different interatomic potentials — the pairwise Morse potential and the many-body potential for Al developed by the embedded atom method. It was found that the use of the many-body potential significantly affects the dynamics of crowdion propagation, including the features of atomic collisions, the evolution of energy localization and the propagation path.

An interaction potential for zinc selenide: A molecular dynamics study

The structural, thermodynamic, and dynamical properties of zinc selenide, as well as the structural phase transformation induced by pressure, based on a many-body interatomic potential that considers two- and three-body interactions, were studied using molecular dynamics simulations. The potential was able to describe the energetics of the zinc-blende, wurtzite, and rock-salt structures of ZnSe. The effective interatomic potential was parameterized using experimental values of the bulk modulus and cohesive energy at an experimental density. Other properties, not used in the parameterization of the potential, such as the vibrational density of states, were correctly described. Cooling from the liquid, an amorphous phase or a re-crystallized material could be obtained. Pair distribution function, coordination number, volume change, and bond angle distributions are presented and compared with available experimental data. The structural phase transition from zinc-blend to rock-salt induced by hydrostatic pressure was obtained at ∼21 GPa for monocrystals and ∼16 GPa for polycrystals.

Basic Structural Units of Tilt Grain Boundaries. II. Misorientation Axes [110] and [111

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries (GB) with the misorientation axes [110] and [111]. The calculations have been carried out with the use of the structural-vacancy model. The study of the atomic structure has been carried out within the entire range of misorientation angles. The reverse density of coincidence sites in special grain boundaries has amounted Σ ≤ 57. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. When calculated with different potentials, the dependence of GB energy on the misorientation angle has a similar form, and the atomic structure completely coincides. It has been shown that the structure of any GB with the misorientation axes [110] and [111] may be represented by a limited number of basic structural units. All found basic structural units defined as units of A, B, C and D types are based on the structures of special grain boundaries. Such special GBs shall be Σ3(111), Σ3(112), Σ11(113) and Σ9(114) for GBs with the misorientation axis [110], and as regarding GBs with the misorientation axis [111], such special GBs shall be Σ3(112), Σ7(123) and Σ13(134). Ranges of angles within which certain basic structural units are found have been defined.

Deep-learning potential method to simulate shear viscosity of liquid aluminum at high temperature and high pressure by molecular dynamics

The shear viscosity of matter and efficient simulating methods in a wide range of temperatures and densities are desirable. In this study, we present the deep-learning many-body potential (the deep potential) method to reduce the computational cost of simulations for the viscosity of liquid aluminum at high temperature and high pressure with accurate results. Viscosities for densities of 2.35 g/cm3, 2.7 g/cm3, 3.5 g/cm3, and 4.27 g/cm3 and temperatures from melting points to about 50 000 K are calculated. The results agree well with the experiment data at a pressure near 1 bar and are consistent with the simulation of first-principles at high pressure and high temperature. We reveal the behavior of the shear viscosity of liquid Al at a range where the current experimental results do not exist. Based on the available experimental data and newly generated simulation data, we propose a modified Enskog–Dymond theory, which can analytically calculate the viscosity of Al at this range. This research is helpful for numerous potential applications.

Molecular dynamics simulation of strengthening of nanocrystalline Cu alloyed with Zr

Nanocrystalline Cu-Zr alloys are an emerging class of immiscible high-strength materials with significant potential for structural and high-temperature applications. The effects of Zr concentration and tensile velocity on the strengthening and mechanics of Cu alloyed with Zr is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that the potential energy of Cu-Zr systems significantly decreases with increasing Zr concentration, resulting in an increase in structural stabilization. Doping Zr atoms at the grain boundaries can strengthen the boundaries and reduce their mobility during the tensile test. The Young’s modulus of Cu-Zr systems decreases with increasing Zr concentration. The plasticity of Cu-Zr systems increases with increasing Zr concentration. The mechanical strength of Cu-Zr systems reaches its maximum value, which is 1.13 times that of the pure Cu counterpart, when the Zr concentration is 5%.

Bottom-up coarse-grained models for external fields and interfaces

Bottom-up coarse-grained (CG) models accurately describe the structure of homogeneous systems but sometimes provide limited transferability and a poor description of thermodynamic properties. Consequently, inhomogeneous systems present a severe challenge for bottom-up models. In this work, we examine bottom-up CG models for interfaces and inhomogeneous systems. We first analyze the effect of external fields upon the many-body potential of mean force. We also demonstrate that the multiscale CG (MS-CG) variational principle for modeling the external field corresponds to a generalization of the first Yvon-Born-Green equation. This provides an important connection with liquid state theory, as well as physical insight into the structure of interfaces and the resulting MS-CG models. We then develop and assess MS-CG models for a film of liquid methanol that is adsorbed on an attractive wall and in coexistence with its vapor phase. While pair-additive potentials provide unsatisfactory accuracy and transferability, the inclusion of local-density (LD) potentials dramatically improves the accuracy and transferability of the MS-CG model. The MS-CG model with LD potentials quite accurately describes the wall-liquid interface, the bulk liquid density, and the liquid-vapor interface while simultaneously providing a much improved description of the vapor phase. This model also provides an excellent description of the pair structure and pressure-density equation of state for the bulk liquid. Thus, LD potentials hold considerable promise for transferable bottom-up models that accurately describe the structure and thermodynamic properties of both bulk and interfacial systems.

Coarse-grained conformational surface hopping: Methodology and transferability.

Coarse-grained (CG) conformational surface hopping (SH) adapts the concept of multisurface dynamics, initially developed to describe electronic transitions in chemical reactions, to accurately describe classical molecular dynamics at a reduced level. The SH scheme couples distinct conformational basins (states), each described by its own force field (surface), resulting in a significant improvement of the approximation to the many-body potential of mean force [T. Bereau and J. F. Rudzinski, Phys. Rev. Lett. 121, 256002 (2018)]. The present study first describes CG SH in more detail, through both a toy model and a three-bead model of hexane. We further extend the methodology to non-bonded interactions and report its impact on liquid properties. Finally, we investigate the transferability of the surfaces to distinct systems and thermodynamic state points, through a simple tuning of the state probabilities. In particular, applications to variations in temperature and chemical composition show good agreement with reference atomistic calculations, introducing a promising "weak-transferability regime," where CG force fields can be shared across thermodynamic and chemical neighborhoods.

Effect of interatomic potential on the sputtering of Pd surfaces

Ion sputtering of surfaces is a widely used technique, for instance utilized to modify surfaces or surface features. Another commonly used technique is the utilization of sputtering to coat another material, where sputtering targets are used to release atoms in order to redeposit them. In self-ion molecular dynamics sputtering simulations, the interatomic potential between the ion and the matrix material is usually the same as for the matrix material. However, for sputtering by ions of different type, the choice is not as self-evident. For most elements, a well-fitted many-body interatomic potential is not available in the literature, and simple pair potentials are used. Quite commonly, the potential between the ion and the matrix is used with no further validation. In this article, we investigate the effect of choice of interatomic potential on the sputtering yield of the sample material palladium by argon ions. We utilize three commonly used pair potential forms, as well as modify these to obtain in total 11 different interatomic potentials for the Ar-Pd interaction. We found that the choice of interatomic potential can drastically affect the low energy sputtering of the surfaces. Additionally, four different surface orientations were simulated to see possible differences. To investigate continuous irradiation, in order to obtain the evolution of surface during prolonged irradiation, cumulative sputtering simulations were carried out.

Hyperdynamics Simulation of the Diffusion of a Vacancy in a Crystal

The diffusion of a vacancy in a deformed aluminum crystal is simulated by the methods of hyperdynamics and classical molecular dynamics. The method used to construct the bias potential for the hyperdynamic method was previously developed for the example of two-dimensional systems using pair potentials. The results indicate the possibility of applying the considered bias potential for the simulation of realistic three-dimensional systems through the use of many-body potentials. The dependences of acceleration of the simulation by the hyperdynamics method in comparison with molecular dynamics on temperature and the bias value of the potential are studied.