UO2

Abstract: We have developed an improved uranium dioxide interatomic potential by fitting to forces, energies, and stresses of first principles molecular dynamics calculations via a genetic algorithm approach called Iterative Potential Refinement (IPR). We compare the defect energetics and vibrational properties of the IPR-fit potential with other interatomic potentials, density functional theory calculations, and experimental phonon dispersions. We find that among previously published potentials examined, there is no potential that simultaneously yields accurate defect energetics and accurate vibrational properties. In contrast, our IPR-fit potential produces both accurate defects and the best agreement with the experimental phonon dispersion and phonon density of states. This combination of accurate properties makes this IPR-fit potential useful for simulating UO2 in high temperature, defect-rich environments typical for nuclear fuel. Additionally, we verify that density functional theory with a Hubbard U correction accurately reproduces the experimentally derived UO2 phonon density of states.

Abstract: We provide a methodology for generating interatomic potentials for use in classical molecular-dynamics simulations of atomistic phenomena occurring at energy scales ranging from lattice vibrations to crystal defects to high-energy collisions. A rigorous method to objectively determine the shape of an interatomic potential over all length scales is introduced by building upon a charged-ion generalization of the well-known Ziegler-Biersack-Littmark universal potential that provides the short- and long-range limiting behavior of the potential. At intermediate ranges the potential is smoothly adjusted by fitting to ab initio data. Our formalism provides a complete description of the interatomic potentials that can be used at any energy scale, and thus, eliminates the inherent ambiguity of splining different potentials generated to study different kinds of atomic-materials behavior. We exemplify the method by developing rigid-ion potentials for uranium dioxide interactions under conditions ranging from thermodynamic equilibrium to very high atomic-energy collisions relevant for fission events.