14 research outputs found
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The Kernel Polynomial Method for non-orthogonal electronic structure calculations
The Kernel Polynomial Method (KPM) has been successfully applied to tight-binding electronic structure calculations as an O(N) method. Here we extend this method to nonorthogonal basis sets with a sparse overlap matrix S and a sparse Hamiltonian H. Since the KPM method utilizes matrix vector multiplications it is necessary to apply S{sup -1} H onto a vector. The multiplication of S{sup -1} is performed using a preconditioned conjugate gradient method and does not involve the explicit inversion of S. Hence the method scales the same way as the original KPM method, i.e. O(N), although there is an overhead due to the additional conjugate gradient part. We show an application of this method to defects in a titanate/platinum interface and to a large scale electronic structure calculation of amorphous diamond
An experimental and computational investigation of structure and magnetism in pyrite CoFeS: Chemical bonding and half-metallicity
Bulk samples of the pyrite chalcogenide solid solutions CoFeS
0 <= x <= 0.5, have been prepared and their crystal structures and magnetic
properties studied by X-ray diffraction and SQUID magnetization measurements.
Across the solution series, the distance between sulfur atoms in the persulfide
(S) unit remains nearly constant. First principles electronic
structure calculations using experimental crystal structures as inputs point to
the importance of this constant S-S distance, in helping antibonding S-S levels
pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half
metal, despite being nearly isostructural and isoelectronic. We use our
understanding of the CoFeS system to make some prescriptions
for new ferromagnetic half-metals.Comment: 8 pages including 9 figure
Bonding in MgSi and AlMgSi Compounds Relevant to AlMgSi Alloys
The bonding and stability of MgSi and AlMgSi compounds relevant to AlMgSi
alloys is investigated with the use of (L)APW+(lo) DFT calculations. We show
that the and phases found in the precipitation sequence are
characterised by the presence of covalent bonds between Si-Si nearest neighbour
pairs and covalent/ionic bonds between Mg-Si nearest neighbour pairs. We then
investigate the stability of two recently discovered precipitate phases, U1 and
U2, both containing Al in addition to Mg and Si. We show that both phases are
characterised by tightly bound Al-Si networks, made possible by a transfer of
charge from the Mg atoms.Comment: 11 pages, 30 figures, submitted to Phys. Rev.