NNSA ASC Exascale Environment Planning, Applications Working Group, Report February 2011

The scope of the Apps WG covers three areas of interest: Physics and Engineering Models (PEM), multi-physics Integrated Codes (IC), and Verification and Validation (V&V). Each places different demands on the exascale environment. The exascale challenge will be to provide environments that optimize all three. PEM serve as a test bed for both model development and 'best practices' for IC code development, as well as their use as standalone codes to improve scientific understanding. Rapidly achieving reasonable performance for a small team is the key to maintaining PEM innovation. Thus, the environment must provide the ability to develop portable code at a higher level of abstraction, which can then be tuned, as needed. PEM concentrate their computational footprint in one or a few kernels that must perform efficiently. Their comparative simplicity permits extreme optimization, so the environment must provide the ability to exercise significant control over the lower software and hardware levels. IC serve as the underlying software tools employed for most ASC problems of interest. Often coupling dozens of physics models into very large, very complex applications, ICs are usually the product of hundreds of staff-years of development, with lifetimes measured in decades. Thus, emphasis is placed on portability, maintainability and overall performance, with optimization done on the whole rather than on individual parts. The exascale environment must provide a high-level standardized programming model with effective tools and mechanisms for fault detection and remediation. Finally, V&V addresses the infrastructure and methods to facilitate the assessment of code and model suitability for applications, and uncertainty quantification (UQ) methods for assessment and quantification of margins of uncertainty (QMU). V&V employs both PEM and IC, with somewhat differing goals, i.e., parameter studies and error assessments to determine both the quality of the calculation and to estimate expected deviations of simulations from experiments. The exascale environment must provide a performance envelope suitable both for capacity calculations (high through-put) and full system capability runs (high performance). Analysis of the results place shared demand on both the I/O as well as the visualization subsystems

Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.

Problems in relativity theory

The dissertation consists of the following three parts: Part I: Mass. Different definitions of mass are discussed. The gravitational mass is shown to be proportional to the inertial mass not only in Newtonian theory but also in relativity theory, when a weak static field is considered. An expression for the mass density of a body in terms of the energy tensor is obtained in the latter case. It is further suggested that the stress components contribute to the mass of the body. Part II: Interpretation of the constants occurring in the solution for a charged particle in general relativity. The constants ([mu] and e), occurring in the solution for a charged particle are usually identified with the mass and the charge of the particle. According to the new interpretation [mu] is found to be the sum of the mass of the central particle and the mass equivalent to the electromagnetic energy of the field. The expression for the density of electromagnetic energy in the relativistic case is twice the corresponding expression in the classical theory. The difference is shown to be a consequence of the fact that the trace of the electromagnetic tensor vanishes. Part III: Negative Mass. Hypothetical netural atoms of negative mass consist of nucleus, with negative mass and positive charge, and electrons, with positive mass and negative charge. On account of the mutual gravitational repulsion, neutral particles of negative mass have a tendency to fly apart. If it is assumed that a body of negative mass emits radiation, it is found that Planck's constant must be negative.<p

Polycrystalline surface properties from spherical crystallites: Ag, Au, Cu and Pt

We have performed a series of atomistic simulations of nearly spherical, crystalline (fcc) clusters of Ag, Au, Cu and Pt as a function of temperature and cluster size. Since both a spherical cluster and a random polycrystal expose all possible surfaces equally, this provides a plausible approach for determining the surface properties of random (non-textured) polycrystalline metals and to find a simple expression to relate these average surface properties to the oft calculated properties of high symmetry/low index surfaces. Atomic clusters with radii greater than approximately 4a0 yield cluster average surface energies and surface stresses are within a few percent of those obtained from very large clusters. The variation of the cluster average surface properties with cluster size is dominated by a geometrical effect associated with the discrete spacing between atomic planes and that the differences associated with differences in the atomic bonding between different elements is small, at least for the four elements considered herein. Comparison of the cluster average surface free energy with those of the more commonly studied high symmetry flat {100}, {110}, and the {111} surfaces suggest two useful approximations for the average surface free energy: (1) equating it to the surface free energy of a {110} surface and (2) using a linear fit to the {100}, {110}, and the {111} surface free energies. Conversely, the first approximation provides an accurate estimate of the {110} surface energy from experimentally measured polycrystalline surface energies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31642/1/0000576.pd

A scalar geodesic deviation equation and a phase theorem

A scalar equation is derived for η, the distance between two structureless test particles falling freely in a gravitational field: η¨+(K−Ω2)η=0. An amplitude, frequency and a phase are defined for the relative motion. The phases are classed as elliptic, hyperbolic and parabolic according as K−Ω2>0,<0,=0. In elliptic phases we deduce a positive definite relative energy E and a phase-shift theorem. The relevance of the phase-shift theorem to gravitational plane waves is discussed