John W. Wilkins

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Silica (SiO(2)) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and(More)
Two modifications of Voter's hyperdynamics scheme offer significant speedup of molecular dynamics simulations. ͑1͒ A simple construction of the bias potential—a few tens of lines of code—is validated for three systems. ͑2͒ A local construction of the bias potential permits the use of intuition to further improve the statistical error. These results suggest(More)
A first-principle quasiparticle theory in the GW approximation is used to compute valence and conduction band offsets, VBO and CBO, respectively, for hexagonal and cubic AlN – GaN interfaces. We find type I band offsets that depend on the in-plane lattice constant of the heterostructure, ranging from VBO ¼ 1:3 eV and CBO ¼ 1:5 eV for the in-plane lattice(More)
Many-body levels of optically excited and multiply charged InAs nanocrystals are studied with the semi-empirical tight-binding model. Single-particle levels of unstrained spherical InAs nanocrystals are described by the sp 3 d 5 s* nearest-neighbor tight-binding model including spin-orbit coupling. For optically excited InAs nanocrystals, first-order(More)
Silicon undergoes a phase transition from the semiconducting diamond phase to the metallic ␤-Sn phase under pressure. We use quantum Monte Carlo calculations to predict the transformation pressure and compare the results to density-functional calculations employing the local-density approximation, the generalized-gradient approximations PBE, PW91, WC, AM05,(More)
We compute the zero frequency current noise numerically and in several limits analytically for the coulomb blockade problem consisting of two tunnel junctions connected in series. At low temperatures over a wide range of voltages, capacitances, and resistances it is shown that the noise measures the variance in the number of electrons in the region between(More)
I mpurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys 1 to steel 2 to planetary cores 3. Experiments and empirical databases are still central to tuning the impurity eff ects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic(More)
Nearly quantitative agreement between density functional theory ͑DFT͒ and diffusion Monte Carlo ͑DMC͒ calculations is shown for the prediction of defect properties using the Heyd-Scuseria-Ernzerhof ͑HSE͒ screened-exchange hybrid functional. The HSE functional enables accurate computations on complex systems, such as defects, where traditional DFT may be(More)
The electron-hole states of semiconductor quantum dots are investigated within the framework of empirical tight-binding descriptions for Si, as an example of an indirect-gap material, and InAs and CdSe as examples of typical III-V and II-VI direct-gap materials. We significantly improve the energies of the single-particle states by optimizing tight-binding(More)
Large-scale simulations of plastic deformation and phase transformations in alloys require reliable classical interatomic potentials. We construct an embedded-atom method potential for niobium as the first step in alloy potential development. Optimization of the potential parameters to a well-converged set of density-functional theory ͑DFT͒ forces,(More)