Bond Order Potentials for AlCuH and C

Al-based Al-Cu alloys have a very high strength to density ratio, and are therefore important materials for transportation systems including vehicles and aircrafts. These alloys also appear to have a high resistance to hydrogen embrittlement, and are therefore being explored for hydrogen related applications. To enable fundamental studies of mechanical behavior of Al-Cu alloys under hydrogen environments, we have developed an Al-Cu-H bond-order potential according to the formalism implemented in the molecular dynamics code LAMMPS. The potential not only fits well properties of a variety of elemental and compound configurations (with coordination varying from 1 to 12) including small clusters, bulk lattices, defects, and surfaces, but also passes the most stringent molecular dynamics simulation tests that sample chaotic configurations. Careful studies verified that this Al-Cu-H potential has unique advantages in terms of giving structural and property trends close to those seen in experiments and quantum-mechanical calculations, and capturing Al-Cu, Al-H, and Cu-H phase diagrams.

Carbon is the most widely studied material today because it exhibits special properties not seen in any other materials when in nano dimensions such as nanotube and graphene. Reduction of material defects created during synthesis has become critical to realize the full potential of carbon structures. Molecular dynamics simulations, in principle, allow defect formation mechanisms to be studied with high fidelity, and can therefore help guide experiments for defect reduction. Such molecular dynamics simulations must satisfy a set of stringent requirements. First, they must employ an interatomic potential formalism that is transferable to a variety of carbon structures. Second, the potential needs to be appropriately parameterized to capture the property trends of important carbon structures, in particular, diamond, graphite, graphene, and nanotubes. Most importantly, the potential must predict the crystalline growth of the correct phases during direct molecular dynamics simulations of synthesis in order to achieve a predictive simulation of defect formation. Because an unlimited number of structures not included in the potential parameterization are encountered, the literature carbon potentials are often not sufficient for growth simulations. We have developed an analytical bond order potential for carbon, and have made it available through the public molecular dynamics simulation package LAMMPS. We demonstrate that our potential reasonably captures the property trends of important carbon phases. Stringent molecular dynamics simulations convincingly show that our potential accounts not only for the crystalline growth of graphene, graphite, and carbon nanotubes, but also for the transformation of graphite to diamond at high pressure.