23 research outputs found
High-performance computing approach to hybrid functionals in the all-electron DFT Code FLEUR
No full textVirtual materials design attempts to use computational methods to discover new materials with superior properties within the vast space of all conceivable materials. Density-functional theory (DFT) is central to this field, enabling scientists to predict material properties from first principles, i.e. without relying on external parameters or experimental values. While standard DFT is capable of predicting many materials with satisfying accuracy, it struggles with some properties such as details of the electronic structure or certain material classes, e.g. materials exhibiting strongly correlated electrons. This has created a need for methods with greater predictive power. One such class of methods are hybrid exchange-correlation functionals which combine the exact Hartree-Fock exchange with local exchange-correlation functionals, resulting in highly accurate predictions for many insulating or semiconductor materials. However, the computational cost of hybrid functionals increases rapidly with system size and limits their application to small systems. This thesis aims to solve the computational challenge posed by hybrid functionals in large systems by utilizing the massive computational power of today's supercomputers. This thesis presents the improved implementation of hybrid exchange-correlation functionals in FLEUR, an all-electron full-potential linearized augmented planewave code. The improved CPU and a new GPU implementations allow users to make efficient use of modern compute nodes and a highly-scalable MPI implementation distributes calculations with a single k-point to 3000 cores or 64 GPUs and far beyond that for calculations with multiple k-points. This work promotes hybrid functionals to systems with hundreds of atoms, opening up their application to many new material classes and properties. We demonstrate the power of this algorithm by applying it to garnets, a class of complex magnetic materials with large unit-cells, which have promising applications in fields such as spintronics or quantum computing. Garnets are rare-earth oxides that exhibit strongly correlated electrons in localized 3d- or 4f-states, making the combination of hybrid functionals and FLAPW ideally suited to investigate these materials. After benchmarking our method against other highly predictive methods and experimental results using yttrium iron garnet as a reference system, we shift our focus the rare-earth-iron garnets gadolinium iron garnet and thulium iron garnet. For these materials we perform the first-ever hybrid exchange-correlation functional calculations of their electronic structure and magnetic moments, establishing the predictive power of the hybrid functionals in FLEUR for large complex magnets
High-Performance Computing Approach to Hybrid Functionals in the All-Electron DFT Code FLEUR
No full textVirtual materials design attempts to use computational methods to discover new materials with superior properties within the vast space of all conceivable materials. Density-functional theory (DFT) is central to this field, enabling scientists to predict material properties from first principles, i.e. without relying on external parameters or experimental values. While standard DFT is capable of predicting many materials with satisfying accuracy, it struggles with some properties such as details of the electronic structure or certain material classes, e.g. materials exhibiting strongly correlated electrons. This has created a need for methods with greater predictive power. One such class of methods are hybrid exchange-correlation functionals which combine the exact Hartree-Fock exchange with local exchange-correlation functionals, resulting in highly accurate predictions for many insulating or semiconductor materials. However, the computational cost of hybrid functionals increases rapidly with system size and limits their application to small systems. This thesis aims to solve the computational challenge posed by hybrid functionals in large systems by utilizing the massive computational power of today’s supercomputer
Interplay of chirality and spin-orbit coupling in the anomalous Hall effect of non-collinear magnets
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Fast All-Electron Hybrid Functionals and Their Application to Rare-Earth Iron Garnets
No full textVirtual materials design requires not only the simulation of a huge number of systems, but also of systems with ever larger sizes and through increasingly accurate models of the electronic structure. These can be provided by density functional theory (DFT) using not only simple local approximations to the unknown exchange and correlation functional, but also more complex approaches such as hybrid functionals, which include some part of Hartree–Fock exact exchange. While hybrid functionals allow many properties such as lattice constants, bond lengths, magnetic moments and band gaps, to be calculated with improved accuracy, they require the calculation of a nonlocal potential, resulting in high computational costs, that scale rapidly with the system size. This limits their wide application. Here, we present a new highly-scalable implementation of the nonlocal Hartree-Fock-type potential into FLEUR—an all-electron electronic structure code that implements the full-potential linearized augmented plane-wave (FLAPW) method. This implementation enables the use of hybrid functionals for systems with several hundred atoms. By porting this algorithm to GPU accelerators, we can leverage future exascale supercomputers which we demonstrate by reporting scaling results for up to 64 GPUs and up to 12,000 CPU cores for a single k-point. As proof of principle, we apply the algorithm to large and complex iron garnet materials (YIG, GdIG, TmIG) that are used in several spintronic applications
Fast hybrid functionals for large systems in the all-electron full-potential linearized augmented plane-wave method FLEUR
No full textGist Perception of Image Composition in Abstract Artworks
No full textMost recent studies in experimental aesthetics have focused on the cognitive processing of visual artworks. In contrast, the perception of formal compositional features of artworks has been studied less extensively. Here, we investigated whether fast and automatic processing of artistic image composition can lead to a stable and consistent aesthetic evaluation when cognitive processing is minimized or absent. To this aim, we compared aesthetic ratings on abstract artworks and their shuffled counterparts in a gist experiment. Results show that exposure times as short as 50 ms suffice for the participants to reach a stable and consistent rating on how ordered and harmonious the abstract stimuli were. Moreover, the rating scores for the 50 ms exposure time exhibited similar dependencies on image type and self-similarity and a similar pattern of correlations between different rating terms, as the rating scores for the long exposure time (3,000 ms). Ratings were less consistent for the term interesting and inconsistent for the term pleasing. Our results are compatible with a model of aesthetic experience, in which the early perceptual processing of the formal aspects of visual artworks can lead to a consistent aesthetic judgment, even if there is no cognitive contribution to this judgment.sponsorship: The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by institute funds from the Institute of Anatomy, University of Jena School of Medicine, to C.R., and long-term structural funding from the Flemish government (METH/14/02) to J.W. (Institute of Anatomy, University of Jena School of Medicine, Flemish government|METH/14/02)status: Publishe
THE FLUORESCENCE SPECTRUM OF NAPHTHALENE VAPOUR
No full textraig, D. P., Hollas, J. M., Redies, M. F. and Wait, S. C. Jr., Proc. Chem. Soc. 1959, p. 361.Author Institution: Division of Pure Physics, National Research CouncilThe fluorescence spectra of naphthalene-h and napthalene-d vapour in the 3100 {\AA} region have been photographed with a high speed spectrograph having a resolution of about . Resonance fluorescence and fluorescence with added helium have been observed. Analysis of the vibronic bands has been attempted using the criterion previously of a single maximum in the rotational contour of a band indicating long axis polarisation of the transition moment (, vibronic symmetry) and a double maximum indicating short axis polarisation ( vibronic symmetry). The existence of two ground-state vibrations of frequencies () and () in naphthalene- and ) and () in naphthalene- will be discussed in relation to the fluorescence spectra
