Highly parallel Monte-Carlo simulations of the acousto-optic effect in heterogeneous turbid media

The development of a highly parallel simulation of the acousto-optic effect is detailed. The simulation supports optically heterogeneous simulation domains under insonification by arbitrary monochromatic ultrasound fields. An adjoint method for acousto-optics is proposed to permit point-source/point-detector simulations. The flexibility and efficiency of this simulation code is demonstrated in the development of spatial absorption sensitivity maps which are in broad agreement with current experimental investigations. The simulation code has the potential to provide guidance in the feasibility and optimization of future studies of the acousto-optic technique, and its speed may permit its use as part of an iterative inversion model

GPAW: open Python package for electronic-structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE) providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation (BSE), variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support of GPU acceleration has been achieved with minor modifications of the GPAW code thanks to the CuPy library. We end the review with an outlook describing some future plans for GPAW

A Study of Potential Organometallic Photosensitizers for TiO2 Using Density Functional Theory

Sunlight is ubiquitous and reliable. Photocatalysis is a promising use for it, with many environmental benefits. One issue with titanium dioxide, a desirable photocatalyst, is an inability to absorb visible light. Attaching organometallics to titanium dioxide may improve photoexcitation. We used density functional theory to model an anatase surface and adsorbates. Our results indicate that carbonyl, as in iron pentacarbonyl and chromium hexacarbonyl, binds poorly to anatase. Halides such as in iron(II) tricarbonyl dibromide improve bonding and reduce required photoexcitation energy. Cyanide, as in tetracyanonickelate and tetracyanopalladate, has greater potential, reducing required energy further. Our results also indicate photocatalysis can be fine-tuned through choice of metal center

Making the most of imaging and spectroscopy in TEM: computer simulations for materials science problems

[eng] Transmission Electron Microscopy (TEM), since its first implementation by Ernst August Friedrich Ruska and Max Knoll in 1931, has been an essential technique in the nanoscience and nanotechnology field. In the beginning, the real resolution was just a small fraction of the potential resolution expected by the fact of using electrons as a “light” source. The wavelength of the electrons accelerated at hundreds of electronvolts would involve a subatomic resolution; however, all the aberrations related to electromagnetic lenses caused a dramatic decrease. In addition, the energy resolution was highly affected by the chromatic aberration of the electron beam. Nowadays, all these initial problems have been solved by the development of the image aberration correctors and the monochromators. Since atomic resolution together with 10 meV energy resolution are a reality for researchers, new and higher horizons have been set for the transmission electron microscopy, such as orbital imaging, phonon imaging, or real time atom monitoring amongst others. TEM could be described at its most fundamental as the analysis of the result of impacting electrons with a specific compound or structure. From, this impact different data can be obtained which can be rapidly classified between imaging and spectroscopy. With the recent increases in energy and spatial resolution, a huge amount of information can be directly extracted from very large experimental datasets; however, for a deeper understanding, most of the times the support from theoretical calculations is also needed. Solid state physics with quantum considerations can contribute to an accurate description of the studied systems. Whereas in the past, materials science, solid state physics, quantum mechanics and chemistry were disciplines with a huge separation between them, nowadays they merge in the field of nanoscience and nanotechnology. When the object size is reduced to the nanoscale the quantum effects cannot be neglected anymore, any change on the synthesis can in turn change the structure which plays an essential role on the compound properties. Thus, modelling has become an essential step in the materials synthesis and characterization. The knowledge of the structure allows to compute the interaction of the electrons with any well described crystalline structure and generate images and spectra comparable with experimental data, but not just as a check, but to gain deeper insight. The interaction of the electrons with matter must be computed by solving the Schrödinger equation of the electrons interacting with the sample. The sample, the system, can be considered as a periodic potential. Imaging, measuring, modelling and manipulating matter are the basis of the promising field of nanoscience, and they can be carried out using a TEM, with the continuous support of theoretical calculations to obtain the most. The present thesis uses three main types of calculations to interpret TEM data: atomic simulations applied to imaging, Boundary Element Method (BEM) based calculations for surface plasmon distributions and Density Functional Theory (DFT) for EELS analysis. Even if they will be presented separately, they are not independent; the essence is always the same but depending on the desired results different considerations are needed. The materials science problems solved through these kinds of simulations presented in the thesis are the analysis of CuPtB ordering effects in GaInP, the influence of oxygen vacancies in the EELS of Bi2O3, the consequences of the Fe3O4 Verwey transition in its electronic structure and how it is observed in EELS and, finally, the surface plasmon distribution in gold-nanodomes as a function of the dome shape. To conclude, the simulations have been presented as an essential tool to complement TEM studies to link the experimental results with the most fundamental aspects which are determined by the structure of the studied materials.[cat] Aquesta tesi doctoral s'ha centrat en la realització de càlculs teòrics que permetin comprendre i extreure la major quantitat d'informació possible de les dades experimentals de microscòpia de transmissió d’electrons (TEM), i de les tècniques espectroscòpiques relacionades, concretament, l'espectroscòpia de pèrdua d’energia dels electrons (EELS). S’hi utilitzen tres tipus principals de càlculs per interpretar les dades del TEM: simulacions atòmiques aplicades a l'obtenció d'imatges, càlculs basats en el mètode d'elements de contorn (BEM) per a les distribucions de plasmons superficials i la teoria del funcional de la densitat (DFT) per a l'anàlisi d’EELS. Tot i que es presentin per separat, no són independents; l'essència sempre és la mateixa, però depenent dels resultats desitjats es necessiten diferents consideracions. En aquest sentit, primerament s'han presentat les bases físiques de diferents mètodes de simulació: simulació multislice per calcular imatges de contrast de número atòmic i de contrast de fase, càlculs (DFT) per calcular dades EELS de baixa pèrdua i de pèrdues profundes i, simulacions basades en BEM per a plasmons de superfície. Un cop presentades les bases, s’han resolt problemes de la ciència dels materials mitjançant aquest tipus de simulacions: l'anàlisi dels efectes d'ordenació del CuPtB al GaInP, la influència de les vacants d'oxigen a l'EELS del Bi2O3, les conseqüències de la transició Fe3O4 Verwey en la seva estructura electrònica i com s'observa a l'EELS i, finalment, la distribució de plasmons superficials als nanodoms d'or en funció de la forma de la cúpula. En resum, al llarg la tesi doctoral les simulacions han demostrat ser una eina essencial per complementar els estudis de TEM, per vincular els resultats experimentals amb els aspectes més fonamentals determinats per l'estructura dels materials estudiats

Space-time multiresolution approach to atomistic visualization

Time-varying three-dimensional positional atomistic data are rich in spatial and temporal information. The problem is to understand them. This work offers multiple approaches that enable such understanding. An interactive atomistic visualization system is developed integrating complex analyses with visualization to present the data on space-time multiresolution basis facilitating the information extraction and generate understanding. This work also shows the usefulness of such an integrated approach. The information obtained from the analyses represents the system at multiple length and time scales. Radial distribution function (RDF) provides a complete average spatial map of the distribution of the atoms in the system which is probed to explore the system at different length scales. Coordination environments and cluster structures are visualized to look at the short range structures. Rings are visualized to understand the medium range structure. Displacement data and covariance matrices are visualized to understand the dynamical behaviors. Combinations of rendering techniques including animation, color map, sphere, polygonal and ellipsoid representations, pathlines and glyphs are used during the visualization process. The three-dimensional atomic configurations are reproduced accurately during rendering because of their physical significance while attributes such as coordination number, coordination stability and atomic species lack direct physical relevance and provide additional flexibilities in rendering. The performance results show interactive frame rates are achievable for systems consisting upto a thousand atoms. Such systems are typical of the systems simulated using first principles molecular dynamics simulations. The effectiveness and the usefulness of this work are justified for complex material systems using silicate and oxide liquids for visual analyses. The exploratory approach taken here has not been reported anywhere else before. The major contributions of this works are: 1. A new approach to the atomistic visualization advocating a formal integration of data analyses into the visualization system to improve the effectiveness and also present an implementation of the exploratory atomistic visualization system with integrated spatio-temporal analytical techniques. 2. The modeling of coordination environments, stability of the coordination environments, clusters, ring structures and diffusion for individual atoms. 3. The use of the visualization system for visual analysis of various liquid mineral systems of geophysical relevance

High-Fidelity Simulation of Small-Body Lander/Rover Spacecraft

The scientific return of spacecraft missions that explore solar system small bodies can be increased through the inclusion of surface exploration with deployed probes. In this dissertation, a methodology is presented that allows for fast, parallel simulation of bouncing trajectories of arbitrary-shaped ballistic probes in the small-body environment. This enables planning of probe deployment and operation, and supports their inclusion on future missions.The coarse small-body shape is modeled using an implicit signed distance field (SDF) that allows for fast collision detection. Statistical features are included onto the SDF using procedural generation techniques. The small-body gravity field is captured using a voxelization of the classical constant-density polyhedron. Surface interactions between a probe and the surface are accounted for using a hard contact model that takes into account restitution and friction. These models are implemented in a GPU environment to allow for the parallel execution of multiple trajectories.The developed simulation framework is applied to perform parametric investigations of probe deployment, which quantify the effects of relevant properties of a probe and its target small body. The probe shape and internal mass distribution are found to strongly affect its deployment dynamics, with near-spherical probes dispersing over greater regions than more distorted shapes. The effect of the surface interactions coefficients on the different shapes variants is quantified. The presence of statistical surface features is also shown to further influence probe dynamics.Finally, the framework is applied to perform a pre-arrival deployment analysis of the MINERVA-II rovers onboard the Hayabusa-2 spacecraft. This analysis identified challenges in the rover deployment and was used to redesign aspects of the nominal rover release sequence. These models will be used to inform the target site selection and follow-on analysis for the Hayabusa-2 mission rover deployments

Commemorative Issue in Honor of Professor Karlheinz Schwarz on the Occasion of His 80th Birthday

A collection of 18 scientific papers written in honor of Professor Karlheinz Schwarz's 80th birthday. The main topics include spectroscopy, excited states, DFT developments, results analysis, solid states, and surfaces