No description available.

Regensburg, Univ., Diss., 2008

Polycalicenes are novel nonbenzenoid aromatic hydrocarbons made from calicene subunits. A host of related polycalicenes are possible by varying the number of calicenes and the bonding motif of the calicenes (e.g. head-to-tail versus head-to-head). Polycalicenes might posses interesting and useful electrical, magnetic, and optical properties. Especially intriguing is that while calicene has never been synthesized, bicalicene 2, where two calicenes are bonded head-to-tail, has been synthesized by Yoshida et al. and found to be aromatic despite having a peripheral 16 ð electron count. This study details a computational investigation of the aromaticity of some planar polycalicenes using the nucleus independent chemical shift (NICS) criterion of aromaticity. NICS values were calculated with HF/6-31+G(d,p) and B3LYP/6-31+G(d,p). Bicalicene 3, where two calicenes are bonded head-to-head, was shown to have a triplet ground state which required the calculation of NICS values using UB3LYP/6-31+G(d,p). Also, the aromaticities of some “belted” polycalicenes were evaluated using NICS values calculated at 6-31G(d,p). The smaller basis set was used due to the increasing number of basis functions for the larger “belted” polycalicenes. In addition, the electronic ground and excited states of the planar polycalicenes were also calculated. The electronic ground states were assigned using HF/6-31+G(d,p) and B3LYP/6-31+G(d,p). CCSD(T)/6-31G(d,p) calculations were used to confirm assignments. The electronic excited states were calculated using time dependent density functional theory (TDDFT) and B3LYP/6-31+G(d,p). Recently, the reliability of the NICS criterion was questioned when it was claimed NICS found the cyclopropenyl anion to be aromatic. One part of this study details the examination of this claim and concluded that the earlier work was in error. The error was proven to arise from the failure to employ diffuse basis functions in the earlier work.

Des calculs ab initio basés sur des méthodes hautement corrélées ont été utilisés pour étudier la structure rovibronique et la spectroscopie des radicaux de 2 à 4 atomes. Pour les molécules CaO, OH, CaH, CaO+ et CaO-, les constantes spectroscopiques calculées sont en accord avec les données expérimentales. L'étude de ces fragments diatomiques est essentielle pour comprendre la structure électronique des isomères du monohydroxyde de calcium (CaOH-HCaO). Le système le plus complexe, parmi ces radicaux diatomiques, est le monoxyde de calcium CaO en raison de la densité élevée des premiers états électroniques. Les énergies rovibrationnelles de ces états ont été calculées sur la base des énergies potentielles obtenues avec un niveau élevé de précision, en tenant compte des couplages entre les moments angulaires et du couplage spin-orbite. Nos résultats sont très proches des résultats expérimentaux. La complexité de l'étude d'un tel système est aussi présente dans le radical ketenyl HCCO à cause de l'effet Renner-Teller qui couple l'état fondamental et le premier état excité. L'effet Renner-Teller dans HCCO a été étudié à l'aide d'une approche variationnelle traitant tous les degrés de liberté. Les coordonnées de valence ont été utilisées pour construire les deux surfaces d'énergie potentielle à six dimensions. Les niveaux rovibroniques les plus bas ont été obtenus pour différentes valeurs du moment angulaire total / Highly correlated ab initio methodologies were used to investigate the rovibronic structure and the spectroscopy of 2 to 4 atoms radicals. For the CaO, OH, CaH, CaO+ and CaO− molecules, our computed spectroscopic constants are in good agreement with the experimental ones. The study of these diatomic fragments is essential to investigate the calcium monohydroxide isomers (CaOH-HCaO). The most complex system among these diatomic radicals is the calcium monoxide radical CaO due to high density low lying electronic states. The rovibrational energies of these states have been calculated, based on the potential energies obtained at high level of accuracy, taking into account the angular momentum and spin orbit couplings and found to be in a very good agreement with the experimental results. The complexity in studying such a system is again present in the ketenyl radical HCCO with highly coupled ground and first excited states. The Renner-Teller effect of HCCO is studied using a variational approach including all degrees of freedom. Valence coordinates have been used to fit both potential energy surfaces varying the six degrees of freedom. The low-lying rovibronic levels have been determined for different values of the total angular momentum

L’étude de réactions chimiques, tout comme le calcul de propriétés thermodynamiques, sont des enjeux capitaux de la chimie moderne. L’évolution des instruments et techniques expérimentales permet des mesures de plus en plus précises de ces grandeurs, pour des systèmes de plus en plus complexes. L’intérêt croissant pour l’étude du milieu interstellaire et des atmosphères planétaires se révèle également être un défi très important dans les décennies à venir. Les difficultés rencontrées lors de l’analyse de ces expériences (ou mesures), nécessitent souvent l’intervention de simulations numériques de manière à éclairer ces observations. Une autre utilisation du calcul est de prédire des paramètres moléculaires et spectroscopiques d’espèces instables difficiles à produire au laboratoire. Les outils actuels de la chimie théorique ab initio sont des moyens précieux pour la prédiction et l’interprétation de résultats expérimentaux ou de mesures astrophysiques et atmosphériques. Ces techniques de simulation ont connu des développements importants au cours des dernières décennies. Les progrès récents en matière de calculs d’interaction de configurations de grande taille permettent d’inclure une grande partie de l’énergie de corrélation. Le temps de calcul et la taille mémoire des ordinateurs restent cependant des limites importantes qui ne permettent pas d’effectuer des interactions de configurations totales dans une base suffisamment grande pour contenir la physique des systèmes étudiés au delà de petites molécules. Cet état de fait conduit à s’intéresser à des méthodes moins coûteuses comme celles des perturbations, les interactions de configurations tronquées et le Coupled Cluster, permettant d’inclure une partie de la corrélation électronique à un coût moins élevé en temps de calcul. Ce sont ces méthodes qui ont été utilisées dans ce travail pour déterminer théoriquement les paramètres moléculaires et spectroscopiques des systèmes MgO, MgO+, FeC2, FeC2+ et FeC2- avec le maximum de précision possible.Dans un premier temps, nous avons étudié la molécule MgO. C’est un système de choix car, il permet de s’initier aux méthodes de calcul ab initio sur les systèmes moléculaires les plus simples (diatomiques), de tester et de comprendre ces méthodes (différentes approximations, validité, précision, …) et de bien interpréter les résultats obtenus (formation de la liaison chimique et des états moléculaires, leur symétrie, leurs couplages, leur stabilité, leur spectroscopie, …) surtout qu’il a fait l’objet de plusieurs études théoriques et expérimentales. Pour profiter de notre savoir-faire pour les molécules diatomiques nous avons étudié le système MgO+ qui a fait l’objet de notre deuxième article que sera présenté en annexe.Dans un second temps, nous avons visé les systèmes moléculaires de type FenCm afin de comprendre la croissance et la dynamique des nanotubes de carbone catalysée par le Fer. Le système diatomique FeC fait l’objet de plusieurs études théoriques et expérimentales. La plus récente est celle fourni par Demeter Tzeli et Aristides Mavridi. Cette étude théorique a caractérisé son état fondamental ainsi que les 40 états électroniques les plus bas, à toutes les distances internucléaires jusqu’à la dissociation, et d’autre part de fournir des données spectroscopiques d’une précision comparable à celle donnée par l’expérience. Pour les systèmes d’ordre supérieur, confronté par le problème que ces petits systèmes moléculaires constitués de Fer et de Carbone ont des structures électroniques très compliquées, notre étude s’est limitée à l’étude des systèmes FeC2, FeC2+ et FeC2- / The study of chemical reactions, as well as the calculation of thermodynamic properties are critical issues of modern chemistry. The development of experimental techniques and instruments allows measurements more accurate these quantities for systems more complex. The growing interest in the study of the interstellar medium and planetary atmospheres is also proving to be a major challenge in the coming decades. The difficulties encountered in the analysis of these experiences (or measures) often require the intervention of numerical simulations to clarify these observations. Another use of the calculation is to predict molecular and spectroscopic parameters of unstable species are difficult to produce in the laboratory.Current tools of theoretical chemistry ab initio are valuable tools for the prediction and interpretation of experimental results or astrophysical measurements and atmospheric. These simulation techniques have experienced significant developments in recent decades. The recent progress in calculations of interaction of large configurations can include a large part of the correlation energy. The computation time and memory size of computers, however, remain significant limitations that do not allow to perform configuration interaction in a total base large enough to hold the physical systems studied beyond small molecules. This fact led to interest in cheaper methods such as disruption, the truncated configuration interaction and coupled cluster, allowing to include a portion of electron correlation at a lower cost in computation time. These are methods that have been used in this work to determine theoretically the molecular parameters and spectroscopic systems MgO, MgO +, FEC2, FEC2 + and FEC2-with maximum accuracy.As a first step, we studied the MgO molecule. It is a system of choice because it allows you to learn the methods of ab initio calculations on molecular systems the simplest (diatomic), test and understand these methods (different approximations, validity, accuracy, ...) and to properly interpret the results (formation of chemical bonding and molecular states, their symmetry, their interactions, their stability, spectroscopy, ...) especially since it has been the subject of several theoretical and experimental studies. To take advantage of our expertise for diatomic molecules we have studied the system MgO + has been our second article will be presented in the appendix.In a second step, we targeted molecular systems FenCm like to understand the growth and dynamics of carbon nanotubes catalyzed by iron. Diatomic system FeC been several theoretical and experimental studies. The most recent is provided by Demeter Tzeli and Aristides Mavridi. This theoretical study has characterized its ground state and the 40 lowest electronic states at all internuclear distances up to dissociation, and secondly to provide spectroscopic data with an accuracy comparable to that given by the experiment. For higher-order systems, the problem faced by these small molecular systems composed of iron and carbon have very complicated electronic structures, our study is limited to the study of systems FEC2, FEC2 + and FEC2-

Recent advances in computational technology and algorithms have made it feasible to accurately model the electronic structures of solids by means of density functional theory. The development of hybrid functionals have improved the accuracy of band gap calculations and made it possible to make qualitative predictions regarding the charge transition energy levels of defects in semiconductors. The Sb-V defect (also known as the E-center) in germanium is a well-known defect, which have been the subject of many experimental and some theoretical studies. It has been found to have interesting annealing properties and the aim of this study is to investigate the electronic properties of the Sb-V defect theoretically. The vacancy defect in germanium (VGe), the antimony substitutional (SbGe) defect in germanium and the defect complex (Sb-V) arising from the combination of these two defects is explored in great detail and how they interact in proximity to one another is presented here. In addition, this work can be seen as a test for the effectiveness of the technique to model defects in semiconductors correctly. The E-center defect was investigated using the HSE06 hybrid functional as implemented in the VASP code. A positive binding energy of 1.5 eV, 1.02 eV and 0.88 eV was found for the first, second and third nearest neighbor configurations respectively, between the Sb and the vacancy was predicted. No metastability was detected and the nearest-neighbor configuration had the lowest energy for all charge states. Four transition levels in the band gap were predicted, with energy level relative to the valence band maximum, lying at 0.52 eV (-2/-1), 0.40 eV (-1/0), 0.44 eV (0/+1) and 0.02 eV (+1/+2). The two mid-gap levels (-1/0) and (0/+1) had negative-U ordering with U= -0.04 eV. iv These findings were consistent with the current experimental model of the Sb-V complex in germanium whereby no metastability has been observed experimentally. The energy level of the (-2/-1) corresponded well with the experimental DLTS level in n-type material at 0.37 eV, though the correspondence for the other levels was not as good. Experimentally, no negative-U behavior was observed, but the predicted negative-U behavior was rather small and no deliberate experiments have been performed to investigate the presence of negative-U behavior in the Sb-V complex. / Dissertation (MSc)--University of Pretoria, 2016. / National Research Foundation (NRF) / Physics / MSc / Unrestricted

Comme de nombreux autres oxydes de métaux de transition lamellaires, les cobaltates dopés au sodium, NaxCoO2, présentent un riche diagramme de phase. Les nombreuses instabilités (magnétiques, de charge) qui les caractérisent seraient notamment le fruit des corrélations électroniques. Dans cette thèse, nous nous intéressons au cas fortement dopé x=2/3 (proche de la limite de l'isolant de bande). Expérimentalement, ce composé est sujet à une disproportion de charge locale sur les atomes de cobalt, ce qui en fait un terrain d'étude privilégié pour le calcul des fonctions de corrélation de charge. Le traitement théorique de ce système est difficile a bien des égards. D'abord, la corrélation électronique demande de recourir à des approximations avancées. Ensuite, le système est sensible aux détails microscopiques, tels que transcrits dans la structure électronique réelle. Dans cette thèse, nous abordons ces deux aspects, à travers un approche sur modèle et une approche ab initio.Nous examinons d'abord l'effet des corrélations au niveau d'un modèle sur réseau triangulaire, pertinent pour les cobaltates. La compétition entre les interactions de Coulomb locale et non-locale sur un modèle de Hubbard étendu donne lieu à des fluctuations de charge, que nous traitons grâce à la théorie du champ moyen dynamique étendue. Nous dressons le diagramme de phase de notre modèle en fonction des interactions locales et non-locales. Celui-ci présente une transition du second ordre entre un état métallique homogène et une phase ordonnée de charge. Nous calculons les observables à une et deux particules dans la phase homogène, et nous déterminons l'effet des corrélations sur ces deux types d'observables. Nous interprétons ces résultats comme des conséquences du fort dopage. Nous identifions une région du diagramme de phase où la partie statique de l'interaction de Coulomb écrantée devient négative. Enfin, nous montrons comment incorporer un terme de Fock non-local à ces calculs, et nous détaillons son effet sur le diagramme de phase et sur les observables physiques.En complément de notre approche sur modèle, nous étudions les détails microscopiques du matériau réel grâce à la théorie de la fonctionnelle de la densité. Nous analysons le rôle de l'hybridation avec l’oxygène et des processus de saut électronique sur la structure de bandes pour les plans de cobalt-oxygène. Nous clarifions l'effet de certains paramètres physiques, tels que le dopage au sodium, l'arrangement cristallin ou le magnétisme, sur la structure électronique. Puis nous calculons la susceptibilité de charge selon les premiers principes, selon des approximations suggérées par notre étude de modèle sur réseau.La comparaison entre les calculs sur modèle et ab initio montre que ces approches se complètent l'une l'autre. D'une part, le modèle permet de dégager les approximations pertinentes pour le calcul des fonctions de corrélation de charge. D'autre part, les calculs ab initio et la compréhension des processus microscopiques sont le préalable à la construction d'un modèle réaliste et prédictif. / As many other layered transition-metal oxides, sodium-doped cobaltates, NaxCoO2, present a rich phase diagram. They display numerous instabilities (magnetic, charge-order), originating most likely from electronic correlations. In this thesis, we focus on the case x=2/3, in the strong-doping limit (close to the band-insulating limit). An experimentally-observed charge disproportionation on the cobalt atoms makes this particular composition an interesting playground to study charge-correlation functions. The theoretical treatment of this system is difficult in several aspects. In order to capture electronic correlations, one needs to resort to advanced non-perturbative approaches. Also, the system is sensitive to its microscopic details, as encoded in the real electronic structure. This is why, in this thesis, we adopt model as well as ab initio approaches to address both these issues.We first study the effect of correlations at the model level, on a triangular lattice, specifically designed for the cobaltates. The interplay between local and non-local Coulomb interactions gives rise to charge fluctuations, which we capture using the Extended Dynamical Mean-Field Theory. We establish the phase diagram of our model as a function of local and non-local interactions. It displays a second-order phase transition between a homogeneous metallic phase, and a charge-ordered phase. We compute one and two-particle observables in the homogeneous phase, and we analyse how they are influenced by correlations. We show that our findings can be understood as a consequence of the strong doping. We find a region of the phase diagram where the static screened Coulomb interaction becomes negative. Finally, we show how to incorporate a non-local Fock term in the calculation of the self-energy, and how it influences the phase diagram and the physical observables.Next, in order to complement the model approach, we study the microscopic details of the real sodium-cobaltates by using Density-Functional Theory. We analyse the effect of oxygen hybridisation and electronic hopping processes on the band structure of cobalt-oxygen planes. We study the effects of sodium doping, crystal structure and magnetism, on the band structure of this material. Then, we compute the charge susceptibility from first principles in the independent-particle approximation and in the random-phase approximation. We use the insight gained from the model calculation to discuss these results.The comparison between the model and the ab initio calculations shows that these approaches are complementary. The model enables us to define the best approximations for the calculation of charge correlation functions. The ab initio calculations, and the detailed understanding of microscopic electronic processes are the prerequisite for a model that is both realistic and predictive.

Nitride fuels have always been considered a good candidate for GENIV reactors, as well as space reactors, due to their high fissile density, highthermal conductivity and high melting point. In these concepts, not beingcompatible with water is not a significant problem. However, in recent years,nitride fuels started to raise an interest for application in thermal reactors,as accident tolerant or high performance fuels. However, oxide fuels havebenefited from decades of intensive research, and thousands of reactor-years.As such, a large effort has to be made on qualifying the fuel and developingtools to help assess their performances.In this thesis, the modeling side of this task is chosen. The effort istwo-fold: determining fundamental properties using atomistic models andputting together all the properties to predict the performances under irradi-ation using a fuel performance code. The first part is done combining manyframeworks. The density functional theory is the basis to compute the elec-tronic structure of the materials, to which a Hubbard correction is added tohandle the strong correlation effects. Negative side effects of the Hubbardcorrection are tackled using the so-called occupation matrix control method.This combined framework is first tested, and then used to find electronic andmechanic properties of the bulk material as well as the thermomechanicalbehavior of foreign atoms. Then, another method, the self-consistent meanfield (SCMF) one, is used to reach the dynamics properties of these foreignatoms. In the SCMF theory, the data that were obtained performing the abinitio simulations are treated to provide diffusion and kinetic flux couplingproperties.In the second step of the work, the fuel performance code TRANSURA-NUS is used to model complete fuel pins. An athermal fission gas releasemodel based on the open porosity is developed and tested on oxide fuels.A model for nitride fuels is introduced, and some correlations are bench-marked. Major issues remaining are pointed out and recommendations asto how to solve them are made. / <p>QC 20170227</p>

Zugl.: Duisburg, Essen, Univ., Diss., 2007

Stuttgart, Univ., Habil.-Schr., 2000.