Accurate structure factors from pseudopotential methods

Highly accurate experimental structure factors of silicon are available in the literature, and these provide the ideal test for any \emph{ab initio} method for the construction of the all-electron charge density. In a recent paper [J. R. Trail and D. M. Bird, Phys. Rev. B {\bf 60}, 7863 (1999)] a method has been developed for obtaining an accurate all-electron charge density from a first principles pseudopotential calculation by reconstructing the core region of an atom of choice. Here this method is applied to bulk silicon, and structure factors are derived and compared with experimental and Full-potential Linear Augmented Plane Wave results (FLAPW). We also compare with the result of assuming the core region is spherically symmetric, and with the result of constructing a charge density from the pseudo-valence density + frozen core electrons. Neither of these approximations provide accurate charge densities. The aspherical reconstruction is found to be as accurate as FLAPW results, and reproduces the residual error between the FLAPW and experimental results.Comment: 6 Pages, 3 figure

Core reconstruction in pseudopotential calculations

A new method is presented for obtaining all-electron results from a pseudopotential calculation. This is achieved by carrying out a localised calculation in the region of an atomic nucleus using the embedding potential method of Inglesfield [J.Phys. C {\bf 14}, 3795 (1981)]. In this method the core region is \emph{reconstructed}, and none of the simplifying approximations (such as spherical symmetry of the charge density/potential or frozen core electrons) that previous solutions to this problem have required are made. The embedding method requires an accurate real space Green function, and an analysis of the errors introduced in constructing this from a set of numerical eigenstates is given. Results are presented for an all-electron reconstruction of bulk aluminium, for both the charge density and the density of states.Comment: 14 pages, 5 figure

Sensitivity analysis of total energy in electronic structure calculations

This work was supported by the Czech Science Foundation, projects no. GA17-12925S and GA17-14840S. The first author acknowledges the support by CEDAMNF project, reg. no. CZ. 02.1.01/0.0/0.0/15 003/0000358, co-funded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDE programme.In the contribution, several approaches to evaluating this term will be analyzed in terms of convergence properties and computing demands using test calculations on simple molecules of nitric oxide, carbon dioxide and tetrafluormethane, see Fig. 1, with interatomic distances scaled by a variable factor

Optimization of pseudopotentials for electronic structure calculations

grant project GA17-12925

Transition vitreuse dans les nanocomposites polyméthacrylate de méthyle - argile organiquement modifiée

Le point de départ de ce travail concerne l'amélioration des performances en service de cylindres de papeterie dont le revêtement est en polyuréthanne. Une brève étude montre que la voie la plus prometteuse est l'introduction de nanocharges lamellaires dont l'effet est d'augmenter la température de transition vitreuse. Devant les résultats contrastés de la littérature et le manque de compréhension des mécanismes mis en jeu, nous avons entrepris une étude plus fondamentale sur un système modèle constitué par des polyméthacrylates de méthyle de stéréorégularité contrôlée et une argile lamellaire organiquement modifiée. En utilisant l'approche calorimétrique de E. Donth, nous mesurons la taille et le nombre de particules concernées dans une région de réarrangement coopératif (CRR) à la transition vitreuse. L'idée maîtresse de notre travail est de faire varier la distance de confinement et d'analyser les effets sur la taille des CRR et la température de transition vitreuse pour des stéréoisomères du PMMA qui présentent des caractères intermoléculaire et intramoléculaire marqués. Nous mettons en évidence que la température de transition vitreuse du polymère confiné dépend de la stéréorégularité, de la masse molaire du polymère, ainsi que de la compétition entre les effets de surface et de taille finie.The starting idea of this work concerns the improvement of polyurethane coated cylinders used in the paper industry. A short study shows that the introduction of nanoclay layers into the polymer matrix is an appealing way to increase the glass transition temperature. The contrasted results published in the literature and the lack of understanding of the involved mechanisms pushed us to undertake a more fundamental study on the behaviour of stereoregular poly (methyl methacrylate) and organo modified clay layers. By using E. Donth's calorimetric approach, we measure the size and the particles number involved in a cooperatively rearranging region (CRR) at the glass transition. The key idea of our work is to vary the confinement distance and to analyse the modification of the size of CRR for stereoregular PMMA with a marked intermolecular and intramolecular character. We show that the glass temperature of the confined polymer depends on the stereoregularity and the chain length but as well on the competition between surface interactions and finite size effects.LORIENT-BU (561212106) / SudocSudocFranceF

BIS Study of Silicon Nitride: Experiment and Theory

X-ray bremsstraMiing isochromat spectroscopic (BIS) study of unoccupied electron states in silicon nitride was performed in an extended energy range. The sample was a 3100 Å layer of silicon nitride deposited on a silicon plate. The isochromat photon energy was 5415 eV. The measurement was performed up to about 250 eV above the BIS threshold. A pronounced maximum of the density of electron state at about lOeV and a weak extended structure up to 200eV were observed. Calculations of the BIS intensity in extended energy range have been performed for silicon nitride using a muffin-tin potential approximation, multiple scattering method and partial probabilities of BIS transitions. The main result is that the BIS of silicon nitride is produced mostly at silicon ions and the BIS extended structure forms mainly due to electron scattering by nearest neighbouring nitrogen ions

Local structure and magnetism of Cu-doped ZnO via CuK-edge XAS and XMCD: theory and experiment

We investigate theoretically the sensitivity of Cu K-edge XANES and XMCD of Cu-doped ZnO (Cu:ZnO) to the local structure. We find that the sensitivity is high enough to enable to distinguish between geometries conceivable for Cu:ZnO. However, one cannot distinguish between Cu and Zn in further coordination shells. Likewise, presence of oxygen vacancies in the ab plane does not alter XANES or x-ray linear dichroism (XLD). By analysing experimental XANES and EXAFS signals of Cu:ZnO pellets (which are paramagnetic) and Cu:ZnO films (which are ferromagnetic) we find that for the pellets, Cu is in Zn-substitutional sites while for the films, the coordination of Cu is as in CuO. Cu K-edge XMCD measurements were performed for both types of samples but detectable signal was obtained only for the pellets. The theory confirms that this XMCD signal originates from magnetic moments localised at Cu atoms located in Zn-substitutional sites