Molecular dynamics simulations using a recently developed Ga-N Tersoff type bond order interatomic potential have been used to investigate the growth mechanisms of ͑0001͒ wurtzite GaN films from thermalized atomic gallium and nitrogen fluxes. The crystallinity and stoichiometry of the deposited wurtzite lattice structures were determined as a function of growth temperature and N:Ga flux ratio. The lattice perfection was found to improve as the growth temperature was increased to 500 K. At a fixed growth temperature, the lattice quality and stoichiometry both reached optimum as the N:Ga ratio approached a value between two and three. The optimum flux ratio increased with increasing growth temperature. These three observations are consistent with experimental studies of growth on wurtzite phase promoting substrates. The atomic assembly mechanisms responsible for these effects have been explored using time-resolved atom position images. The analysis revealed that high quality crystalline growth only occurred when off-lattice atoms ͑which are usually associated with amorphous embryos or defect complexes͒ formed during deposition were able to move to unoccupied lattice sites by thermally activated diffusion processes. The need for a high N:Ga flux ratio to synthesize stochiometric films arises because many of the nitrogen adatoms that impact N-rich ͑0001͒ GaN surfaces are re-evaporated. Reductions of the substrate temperature reduce this reevaporation and as a result, the optimum N:Ga ratio for the stoichiometric film formation ͑and best lattice perfection͒ was reduced as the growth temperature was decreased.

The tight-binding description of covalent bonding is used to propose a four-level, bond-order potential for elemental silicon. The potential addresses both the and bonding and the valence of this sp-valent element. The interatomic potential is parametrized using ab initio and experimental data for the diamond cubic, simple cubic, face-centered-cubic, and body-centered-cubic phases of silicon. The bond-order potential for silicon is assessed by comparing the predicted values with other estimates of the cohesive energy, atomic volume, and bulk modulus for the ␤-Sn, bc8, st12, and 46 clathrate structures. The potential predicts a melting temperature of 1650± 50 K in good agreement with the experimental value of 1687 K. The energetics of various highsymmetry point defect structures and the structure and energetics of small silicon clusters are investigated. The potential also provides a robust description of surface reconstructions; it notably predicts with high fidelity the surface formation energy of the ͑111͒ 7 ϫ 7 dimer adatom stacking fault configuration.

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Recent mapping of flows of the Columbia River Basalt Group between Lewiston and Pomeroy, southeast Washington, places the chemically distinctive Shumaker Creek flow as a new member between the Frenchman Springs and Roza members of the Wanapum Basalt. This leaves the Eckler Mountain Formation composed of only the Robinette Mountain and Dodge chemical types, with the Lookingglass flow forming the base of the overlying Wanapum Basalt. One Robinette Mountain flow and five separate flows of Dodge composition are recognized and traced across the Blue Mountains Anticline of southeast Washington and northeast Oregon. The aerial distribution of the flows is used to constrain the onset of deformation in the Blue Mountains area between the Hite and Limekiln faults. A series of open east–west folds formed during late Wanapum and Saddle Mountains time, cut by northeast-trending faults with left-lateral strain. Chemical variations between Eckler Mountain, Grande Ronde, and Wanapum Basalt flows require different source components. But between the Eckler Mountain flows the variation of most chemical parameters is consistent with fractional crystallization in the crust and can be modeled for major and trace elements. An exception is the behaviour of Cr and Zr/Y between the Robinette Mountain and Dodge flows, which suggests variable partial melting or possibly olivine accumulation.

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