[en] An efficient new method is presented to calculate the quantum transports using periodic boundary conditions. This method allows the use of conventional ground state ab initio programs without big changes. The computational effort is only a few times of a normal groundstate calculations, thus is makes accurate quantum transport calculations for large systems possible

[en] The electronic structure of a GaAsN alloy is calculated using a 4096 atom supercell, with a 70 Ry plane wave basis cutoff and Ga atom 3d electrons as valence electrons. The charge density of this supercell is generated by patching the charge density of a small unit cell with the charge density of bulk GaAs. The local-density-approximation band gap error is corrected by modifying the nonlocal pseudopotentials. A localized nitrogen state [a₁(N)] is obtained,and it plays an important role in the band gap reduction of GaAsN

No abstract available

[en] The electronic structures of cubic InGaN systems are calculated using an atomistic empirical pseudopotential method. Two extreme cases are studied. One is a pure InN quantum dot embedded in a pure GaN matrix, another is a pure In_xGa_1-xN alloy without clustering. We find hole localizations in both cases. The hole wave function starts to be localized as soon as a few In atoms segregate to form a small cluster, while the electron wave function only becomes localized after the number of In atoms in the quantum dot becomes larger than 200. The hole state is also strongly localized in a pure In_xGa_1-xN alloy, on top of randomly formed (110) directioned In-N-In chains. Using one proposed model, we have calculated the hole energy fluctuation, and related that to photoluminescence linewidth. The calculated linewidth is about 100 meV, close to the experimental results. Wurtzite InGaN is also studied for optical anisotropies. We find that in both quantum dot and pure alloy, the polarization is in the xy plane perpendicular to the c axis of the wurtzite structure

No abstract available

[en] Quantum mechanical ab initio calculation constitutes the biggest portion of the computer time in material science and chemical science simulations. As a computer center like NERSC, to better serve these communities, it will be very useful to have a prediction for the future trends of ab initio calculations in these areas. Such prediction can help us to decide what future computer architecture can be most useful for these communities, and what should be emphasized on in future supercomputer procurement. As the size of the computer and the size of the simulated physical systems increase, there is a renewed interest in using the real space grid method in electronic structure calculations. This is fueled by two factors. First, it is generally assumed that the real space grid method is more suitable for parallel computation for its limited communication requirement, compared with spectrum method where a global FFT is required. Second, as the size N of the calculated system increases together with the computer power, O(N) scaling approaches become more favorable than the traditional direct O(N³) scaling methods. These O(N) methods are usually based on localized orbital in real space, which can be described more naturally by the real space basis. In this report, the author compares the real space methods versus the traditional plane wave (PW) spectrum methods, for their technical pros and cons, and the possible of future trends. For the real space method, the author focuses on the regular grid finite different (FD) method and the finite element (FE) method. These are the methods used mostly in material science simulation. As for chemical science, the predominant methods are still Gaussian basis method, and sometime the atomic orbital basis method. These two basis sets are localized in real space, and there is no indication that their roles in quantum chemical simulation will change anytime soon. The author focuses on the density functional theory (DFT), which is the most used method for quantum mechanical material science simulation

[en] Unconventional semiconductor alloys exhibit many unusual features and are under intensive studies recently. However, as initio methods cannot be applied directly to these systems due to their large sizes. In this work, a motif based charge patching method is introduced to generate the ab initio quality charge densities for these large systems. The resulting eigen energies are almost the same as the original ab initio eigen energies (with 20-50 meV errors)

[en] Recent experiments have indicated that 3-mercapto-1-propanolligands display a size-dependent binding energy of attachment to the surface of II-VI semiconductor nanocrystals. Using semi-empirical calculations, we exhaustively calculate the energy of this bond at each surface site, for CdSe and CdSe/CdS core/shell nanocrystals ranging from1.8 to 4.1 nm in diameter. Our results suggest that the experimentally observed changes in binding energy are due to the distribution of surface facets on the nanocrystals, and not related to the band gap, as proposed in the experimental paper

[en] Isoelectronic impurity states are localized states induced by stoichiometric single atom substitution in bulk semiconductor. Photoluminescence spectra indicate deep impurity levels of 0.5 to 0.9eV above the top of valence band for systems like: GaN:As, GaN:P, CdS:Te, ZnS:Te. Previous calculations based on small supercells seemingly confirmed these experimental results. However, the current ab initio calculations based on thousand atom supercells indicate that the impurity levels of the above systems are actually much shallower(0.04 to 0.23 eV), and these impurity levels should be compared with photoluminescence excitation spectra, not photoluminescence spectra

[en] We study the structure of the energy spectra along with the character of the states participating in optical transitions in colloidal CdS quantum dots (QDs) using the ab initio accuracy charge patching method combined with the folded spectrum calculations of electronic structure of thousand-atom nanostructures. In particular, attention is paid to the nature of the large resonant Stokes shift observed in CdS quantum dots. We find that the top of the valence band state is bright, in contrast with the results of numerous k · p calculations, and determine the limits of applicability of the k · p approach. The calculated electron-hole exchange splitting suggests the spin-forbidden valence state may explain the nature of the ''dark exciton'' in CdS quantum dots