Materials and Molecular Modelling at the Exascale

Progression of computational resources towards exascale computing makes possible simulations of unprecedented accuracy and complexity in the fields of materials and molecular modelling (MMM), allowing high fidelity in silico experiments on complex materials of real technological interest. However, this presents demanding challenges for the software used, especially the exploitation of the huge degree of parallelism available on exascale hardware, and the associated problems of developing effective workflows and data management on such platforms. As part of the UKs ExCALIBUR exascale computing initiative, the UK-led MMM Design and Development Working Group has worked with the broad MMM community to identify a set of high priority application case studies which will drive future exascale software developments. We present an overview of these case studies, categorized by the methodological challenges which will be required to realize them on exascale platforms, and discuss the exascale requirements, software challenges and impact of each application area

Experimental and density functional study of Mn doped Bi₂Te₃ topological insulator

We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure show that Mn has valency of nominally 2+. Extended x-ray absorption fine structure spectroscopy in combination with electron energy loss spectroscopy (EELS) shows that Mn is a substitutional dopant of Bi and Te and also resides in the van der Waals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanningtransmission electron microscopy and EELS show that Mn substitution of Te occurs in film regions with increased Mn concentration. First-principles calculations show that the Mn dopants favor octahedral sites and are ferromagnetically coupled

Electron and vibrational spectroscopies using DFT, plane waves and pseudopotentials: CASTEP implementation

Density functional theory can be used to interpret and predict spectroscopic properties of solid-state materials. The relevant computational solutions are usually available in disparate DFT codes, so that it is difficult to use a consistent approach for analyzing various spectroscopic features of a given material. We review the latest developments that are aimed to provide a collection of analytical tools within one DFT package, CASTEP. The applications covered include core-level EELS, solid-state NMR, optical properties, IR and Raman spectroscopy. We present also results of the EELS analysis of NbO and Nb 2O 5 that show the first published example of CASTEP spectra from d-states. Raman activities calculated for a test set of small molecules and the convergence requirements for such calculations are discussed. © 2010 Elsevier B.V