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Modeling the dynamics of interacting many-particle systems is one of the central challenges at the forefront of condensed matter physics. When systems of interacting particles are driven out of equilibrium, complex emergent behaviors can arise that are difficult to predict from their individual properties. In this work, we model two such dynamic many-particle systems. The first is a nanocomposite of conductive CrO2 and insulating Cr2O3 nanoparticles. We numerically model the nanoparticles as hard spherocylinders, systems of which are compressed into dense, disordered, packings via the mechanical contraction method, with various volume fractions of conductive CrO2. We then analyze the resulting ensembles of these nanocomposites to identify their critical percolation properties through a finite size scaling analysis. We use a randomly walking ”blind ant” approach to calculate the conductivity of the nanocomposite and obtain the conductivity critical exponent of the nanocomposite system, which agrees with experimental measurements. Intriguingly, the calculated percolation threshold we obtained, pc= 0.312 ± 0.002, is near (within numerical errors) those found previously in two other systems, disordered jammed spheres, and simple cubic lattice.The second type of system we model the evolution of is embedded grains in two dimensional (2D) crystals. We model 2D materials with the phase field crystal (PFC) model, which captures both atomic spatial resolution and slow diffusive dynamics, allowing for precise mapping of defect structures around evolving grain boundaries. We apply the Cahn-Taylor formulation of grain boundary motion to show that the normal motion of a shrinking embedded grain boundary couples to the tangential motion of atoms along the boundary, resulting in a net rotation of the grain as it changes size. Furthermore, we show that a more complex two component system such as hexagonal boron nitride (h-BN) exhibits a dual mode behavior of grain rotation, where the bonding energy difference between different atomic species results in competing rotations in opposite directions for binary embedded grains. This highlights the role played by the lattice inversion symmetry breaking in binary or multi-component materials as compared to single-component materials. The potential implications for processing techniques used to produce large crystals of 2D materials are also discussed.

The dynamical mechanisms underlying the grain evolution and growth are of fundamental importance in controlling the structural properties of large-scale polycrystalline materials, but the effects of lattice ordering and distinct atomic species in multi-component material systems are still not well understood. We study these effects through the phase field crystal modeling of embedded curved grains in two-dimensional hexagonal materials, by examining and comparing the results of grain rotation, shrinking, and grain boundary dynamics over the full range of misorientation in binary systems of hexagonal boron nitride and single-component graphene monolayers. Calculations of the relation between grain radius and misorientation angle during time evolution reveal the normal-tangential coupled motion of the grain boundary matching the Cahn-Taylor formulation, as well as the transition to sliding and the regime of grain motion without rotation. The key effect of two-component sublattice ordering is identified, showing as a dual behavior of both positive and negative coupling modes with grains rotating towards increasing and decreasing angles, which is absent in two-dimensional single-component systems. The corresponding mechanisms are beyond the purely geometric considerations and require the energetic contribution from the difference between heteroelemental and homoelemental atomic bondings and the subsequent availability of a diverse variety of defect core structures and transformations. This indicates the important role played by the lattice inversion symmetry breaking in binary or multi-component materials, causing the change of detailed microstructures and dynamics of dislocation defects at grain boundaries as compared to single-component materials.

We present a systematic methodology, built within the Open Knowledgebase of Interatomic Models (OpenKIM) framework (https://openkim.org), for quantifying properties of grain boundaries (GBs) for arbitrary interatomic potentials (IPs), GB character, and lattice structure and species. The framework currently generates results for symmetric tilt GBs in cubic materials, but can be readily extended to other types of boundaries. In this paper, GB energy data are presented that were generated automatically for Al, Ni, Cu, Fe, and Mo with 225 IPs; the system is installed on openkim.org and will continue to generate results for all new IPs uploaded to OpenKIM. The results from the atomistic calculations are compared to the lattice matching model, which is a semi-analytic geometric model for approximating GB energy. It is determined that the energy predicted by all IPs (that are stable for the given boundary type) correlate closely with the energy from the model, up to a multiplicative factor. It thus is concluded that the qualitative form of the GB energy versus tilt angle is dominated more by geometry than the choice of IP, but that the IP can strongly affect the energy level. The spread in GB energy predictions across the ensemble of IPs in OpenKIM provides a measure of uncertainty for GB energy predictions by classical IPs.

While classical percolation is well understood, percolation effects in randomly packed or jammed structures are much less explored. Here we investigate both experimentally and theoretically the electrical percolation in a binary composite system of disordered spherocylinders, to identify the relation between structural (percolation) and functional properties of nanocomposites. Experimentally, we determine the percolation threshold \\(p_c\\) and the conductivity critical exponent \\(t\\) for composites of conducting (CrO\\(_2\\)) and insulating (Cr\\(_2\\)O\\(_3\\)) rodlike nanoparticles that are nominally geometrically identical, yielding \\(p_c=0.305 0.026\\) and \\(t=2.52 0.03\\) respectively. Simulations and modeling are implemented through a combination of the mechanical contraction method and a variant of random walk (de Gennes ant) approach, in which charge diffusion is correlated with the system conductivity via the Nernst-Einstein relation. The percolation threshold and critical exponents identified through finite size scaling are in good agreement with the experimental values. Curiously, the calculated percolation threshold for spherocylinders with an aspect ratio of 6.5, \\(p_c=0.312 0.002\\), is very close (within numerical errors) to the one found previously in two other distinct systems of disordered jammed spheres and simple cubic lattice, an intriguing and surprising result.

The Open Knowledgebase of Interatomic Models (OpenKIM) is an NSF Science Gateway that archives fully functional computer implementations of interatomic models (potentials and force fields) and simulation codes that use them to compute material properties. Interatomic models are coupled with compatible simulation codes and executed in a fully automated manner by the OpenKIM processing pipeline, a cloud-based computation platform. The pipeline as previously introduced in the literature was insufficient to support the large-scale computations that have become necessary within the materials science community. Accordingly, we present extensions made to the pipeline that allow it to utilize High-Performance Computing (HPC) resources in an efficient and performant fashion.

New functionality is added to the LAMMPS molecular simulation package that increases the versatility with which LAMMPS can interface with supporting software andmanipulate information associated with bonded force fields. We introduce the “type label” framework that allows atom types and their higher-order interactions (bonds, angles, dihedrals, and impropers) to be represented in terms of the standard atom type strings of a bonded force field. Type labels increase the human readability of input files, enable bonded force fields to be supported by the OpenKIM repository, simplify the creation of reaction templates for the REACTER protocol, and increase compatibility with external visualization tools such as VMD and OVITO. An introductory primer on the forms and use of bonded force fields is provided to motivate this new functionality and serve as an entry point for LAMMPS and OpenKIM users unfamiliar with bonded force fields. The type label framework has the potential to streamline modeling workflows that use LAMMPS by increasing the portability of software, files, and scripts for pre-processing, running, and post-processing a molecular simulation.