<p>Fullerenes have been shown to constitute a prototypical building block for truly nanometer-sized devices and exotic nanounit-based materials, e.g., high-temperature superconductors. This makes the detailed understanding of fullerene electronic states in compounds and at interfaces of primary importance, since the high symmetry of the molecule greatly </p><p>simplifies the starting point of the analysis. Carbon nanotubes, which combine one macroscopic with two nanoscopic dimensions, are perhaps of even greater practical interest.</p><p>Angle-dependent electron spectroscopies have been employed in the present work to study these materials, characterizing their structure, bonding, and electronic states. For solid C₆₀, the photoelectron angular distribution has been found to be essentially that of the free molecule, modified by solid state scattering; a similar distribution is found for K₃C_60.</p><p>The surface and bulk electronic structure of K₃C₆₀ has been identified by angle-dependent core and valence photoelectron spectroscopy (PES) and x-ray emission spectroscopy. An insulating surface layer has been identified for this high-temperature superconductor.</p><p>Angle-dependent valence PES is used to investigate the electronic states of C₆₀/Al(110). Electron correlations are found to be the origin of the splitting observed in the molecular orbitals, which is quite sensitive to the molecular orientation. The components of the highest occupied molecular orbital are differentiated according to their overlap with the substrate.</p><p>A rigid shift of valence- and core-levels has been observed even for ionic and covalent C₆₀compounds, reflecting the efficient static polarizability screening of the molecule. </p><p>The alignment of multi-walled carbon nanotubes has been investigated by x-ray absorption spectroscopy, using the spectral intensity ratio of π*- and *-resonances. Core level combined with valence PES shows that the degree of defect structure varies from position to position on the sample. Valence photoelectron spectra of defect-free sample spots closely resembles the total DOS of graphite.</p>

Fullerenes have been shown to constitute a prototypical building block for truly nanometer-sized devices and exotic nanounit-based materials, e.g., high-temperature superconductors. This makes the detailed understanding of fullerene electronic states in compounds and at interfaces of primary importance, since the high symmetry of the molecule greatly simplifies the starting point of the analysis. Carbon nanotubes, which combine one macroscopic with two nanoscopic dimensions, are perhaps of even greater practical interest. Angle-dependent electron spectroscopies have been employed in the present work to study these materials, characterizing their structure, bonding, and electronic states. For solid C60, the photoelectron angular distribution has been found to be essentially that of the free molecule, modified by solid state scattering; a similar distribution is found for K3C60. The surface and bulk electronic structure of K3C60 has been identified by angle-dependent core and valence photoelectron spectroscopy (PES) and x-ray emission spectroscopy. An insulating surface layer has been identified for this high-temperature superconductor. Angle-dependent valence PES is used to investigate the electronic states of C60/Al(110). Electron correlations are found to be the origin of the splitting observed in the molecular orbitals, which is quite sensitive to the molecular orientation. The components of the highest occupied molecular orbital are differentiated according to their overlap with the substrate. A rigid shift of valence- and core-levels has been observed even for ionic and covalent C60 compounds, reflecting the efficient static polarizability screening of the molecule. The alignment of multi-walled carbon nanotubes has been investigated by x-ray absorption spectroscopy, using the spectral intensity ratio of π*- and *-resonances. Core level combined with valence PES shows that the degree of defect structure varies from position to position on the sample. Valence photoelectron spectra of defect-free sample spots closely resembles the total DOS of graphite.

Extensive theoretical studies on the ground and excited state properties of systems containing heavy atoms have shown that accurate prediction of transition energies and related properties requires the incorporation of both relativistic and higher order correlation and relaxation effects as these effects are strongly inter- wined. The relativistic and dynamical electron correlation effects can be incor- porated in many-electron systems through a variety of many-body methods like configuration interaction (CI), coupled cluster method (CCM) etc. which are very powerful and effective tool for high precision description of electron correlation in many-electron systems. In this thesis, we investigate the relativistic and correlation effects in heavy atomic and molecular systems using these two highly correlated many-body methods. It is well recognized that, heavy polar diatomic molecules such as BaF, YbF, TlF, PbO, etc. are the leading experimental candidates for the search of violation of Parity (P ) and Time-reversal (T ) symmetry. The experimental detection of such P,T-odd effects in atoms and molecules has important consequences for the theory of fundamental interactions or for physics beyond the standard model (SM). For instance, a series of experiments on TlF have already been reported which provide the tightest limit available on the tensor coupling constant C , proton electric dipole moment (EDM) dp , etc. Experiments on YbF and BaF molecules are also of fundamental significance to the study of symmetry violation in nature, as these experiments have the potential to detect effects due to the electron EDMde. It is therefore imperative that high precession calculations are necessary to interpret these ongoing (and perhaps forthcoming) experimental outcome. For example, the knowledge of the effective electric field E(characterized by Wd) at the unpaired electron is required to link the experimentally determined P,T-odd frequency shift with the electron EDM de. We begin with a brief review of P,T-odd effects in heavy atoms and polar diatomics and the possible mechanisms which can give rise to such effects, in particular, the one arises due to the intrinsic electron EDM de. The P,T-odd interaction constant Wd is computed for the ground (2∑ ) state of YbF and BaF molecules using all-electron DF orbitals at the restricted active space (RAS) CI level. The RASCI space used for both systems in this calculation is sufficiently large to incorporate important core-core, core-valence, and valence-valence electron correlation effects. In addition to Wd, we also report the dipole moment (µe ) for these systems to assess the reliability of the method. The basis set dependency of Wd is also analyzed. The single reference coupled cluster (SRCC) method, developed by the cluster expansion of a single determinant reference function, is one of the most sophisticated method for treating dynamical correlation effects in a size-extensive manner. The non-uniqueness of the exponential nature of the wave operator diversifies the methods in multi-reference context. The multi-reference coupled cluster (MRCC) strategies fall within two broad classes: (a) State-Universal (SU), a Hilbert-space approach and (b) Valence-Universal (VU), a Fock-space approach. In this thesis, we shall be mainly concerned with the VU-MRCC which unlike SU-MRCC uses a single wave operator that not only correlates the reference functions, but also all the lower valence (or the so called subdued) sectors, obtained by deleting the occupancies systematically. The linear response theory (LRT) or equation of motion (EOM) method is another possible alternative which is nowadays extensively used to compute the atomic and molecular properties. Although, the CCLRT or EOM-CC method is not fully extensive in nature, this method has some distinct advantages over the traditional VU-MRCC theory. Further, for one-valence problem like ionization processes, the CCLRT/EOM-CC is formally equivalent to VU-MRCC, and hence, size-extensive. In this thesis, the core-extensive CCLRT and core-valence extensive (all electron) VU-MRCC methods are applied to compute the ground and excited state properties of various atomic and molecular systems (HCl, CuH, Ag, Sr, Yb and Hg) using nonrelativistic and relativistic (for heavy atoms) spinors. The similarities and differences in the structure of these two formalisms are also addressed. We also investigate the ground and excited state properties of HCN which is a system of astrophysical importance. This system has raised interest among the astrophysicists due to its detection in the atmosphere of Titan and Carbon stars. HCN has also been identified via radio-techniques in both comets and interstellar atmosphere. In the ash-photolysis of oxazole, iso-oxazole, and thiozole a transient band system was observed in the region 2500-3050 Å. This band system was attributed to a meta-stable form of HCN, i.e, either HNC or triplet HCN. We carry out detailed theoretical investigations using CCLRT and complete active space self-consistent field (CASSCF) method to characterize this unidentified band and other experimentally observed transitions.

Submitted by Cláudia Bueno (claudiamoura18@gmail.com) on 2015-10-29T18:35:28Z No. of bitstreams: 2 Tese - Jose Higino Damasceno Junior - 2015.pdf: 2058291 bytes, checksum: ed4c947cd5e0f908dddc93570ac84dbb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-11-03T14:21:33Z (GMT) No. of bitstreams: 2 Tese - Jose Higino Damasceno Junior - 2015.pdf: 2058291 bytes, checksum: ed4c947cd5e0f908dddc93570ac84dbb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-11-03T14:21:33Z (GMT). No. of bitstreams: 2 Tese - Jose Higino Damasceno Junior - 2015.pdf: 2058291 bytes, checksum: ed4c947cd5e0f908dddc93570ac84dbb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-09-25 / Fundação de Amparo à Pesquisa do Estado de Goiás - FAPEG / Clusters systems are very different from molecules or their bulk materials, since they exhibit many specific properties. As example, the bond in metallic clusters of metallic atoms is intermediate between metallic and covalent bonding. In general, the structural and electronic properties of these systems are very difficult to measure experimentally, and therefore theoretical modeling is very important in characterizing them. In this thesis, we employed ab initio methods to study metallic clusters such as the aluminum hydride clusters as well as a few aromatic metal clusters. The optimized geometries of the studied clusters have been determined using DFT. The electronic structures of these systems were investigated using the QMC methods. The calculations were carried out within the Variational (VMC) and fixed-node diffusion (DMC) quantum Monte Carlo methods. The calculations are also performed in the Hartree-Fock (HF) approximation in order to analyze the impact of electron correlation. With regards the aluminum hydride clusters, the total atomic binding energy impact varies from ~20% up to about ~50%, whereas for the electron binding energy it ranges from ~1% up to ~73%. The decomposition of the electron binding energies clearly shows that both charge redistribution and electron correlation are important in determining the detachment energies, whereas electrostatic and exchange interactions are responsible for the ionization potential. For the aromatic metal clusters, the presence of a dopant plays important role in their electronic properties enhancing their binding energy, electron affinity, hardness and resonance energy. / Clusters são sistemas bastante diferentes de moléculas e sólidos, pois exibem propriedades bastante peculiares. Por exemplo, a ligação em um cluster metálico tem intensidade intermediária entre as ligações covalentes e metálicas. Em geral, as propriedades eletrônicas e estruturais desses sistemas são bastante difíceis de serem medidas experimentalmente e, portanto, uma modelagem teórica é muito importante na caracterização desses. Nesta Tese, utilizamos métodos ab initio para estudar clusters metálicos, tal como clusters de hidretos de alumínio assim como também alguns clusters metálicos aromáticos. As estruturas geométricas dos clusters estudados foram otimizadas via DFT. A estrutura eletrônica desses clusters foi investigada usando o método de Monte Carlo Quântico Variacional (MCQD) e de difusão (MCQD) com aproximação de nós fixos. Os cálculos também foram realizados a partir da aproximação de Hartree-Fock, afim de se analisar o impacto da energia de correlação eletrônica. Para os hidretos de alumínio, a energia de correlação eletrônica tem impacto na energia total de ligação variando de 20% a 50%. Da mesma maneira, a energia de ligação de um elétron ao cluster tem grande contribuição da energia de correlação eletrônica, variando de 1% a 73%. A decomposição da energia de ligação mostra claramente que a relaxação e a correlação eletrônica são importantes na determinação da afinidade eletrônica, enquanto que a interação de troca eletrostática é responsável pelo potencial de ionização. Para os clusters aromáticos, a presença do dopante desempenha um importante papel nas propriedades desses clusters, uma vez que otimiza a energia de ligação, a afinidade eletrônica, a dureza e a energia de ressonância.

The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

In this thesis, the implementation of the incremental scheme for open-shell systems with unrestricted Hartree-Fock reference wave functions is described. The implemented scheme is tested within robustness and performance with respect to the accuracy in the energy and the computation times. New approaches are discussed to implement a fully automated incremental scheme in combination with the domain-specific basis set approximation. The alpha Domain Partitioning and Template Equalization are presented to handle unrestricted wave functions for the local correlation treatment. Both orbital schemes are analyzed with a test set of structures and reactions. As a further goal, the DSBSenv orbital basis sets and auxiliary basis sets are optimized to be used as environmental basis in the domain-specific basis set approach. The performance with respect to the accuracy and computation times is analyzed with a test set of structures and reactions. In another project, a scheme for the optimization of auxiliary basis sets for uranium is presented. This scheme was used to optimize the MP2Fit auxiliary basis sets for uranium. These auxiliary basis enable density fitting in quantum chemical methods and the application of the incremental scheme for systems containing uranium. Another project was the systematical analysis of the binding energies of four water dodecamers. The incremental scheme in combination with the CCSD(T) and CCSD(T)(F12*) method were used to calculate benchmark energies for these large clusters.

In dieser Dissertation werden ab initio Theorien zur Beschreibung der Zustände von perfekten halbleitenden und nichtleitenden Kristallen, unter Berücksichtigung elektronischer Korrelationen, abgeleitet und angewandt. Als Ausgangsbasis dient hierzu die Hartree-Fock Approximation in Verbindung mit Wannier-Orbitalen. Darauf aufbauend studiere ich zunächst in Teil I der Abhandlung den Grundzustand der wasserstoffbrückengebundenen Fluorwasserstoff und Chlorwasserstoff zick-zack Ketten und analysiere die langreichweitigen Korrelationsbeiträge. Dabei mache ich die Basissatzextrapolationstechniken, die für kleine Moleküle entwickelt wurden, zur Berechnung von hochgenauen Bindungsenergien von Kristallen nutzbar. In Teil II der Arbeit leite ich zunächst eine quantenfeldtheoretische ab initio Beschreibung von Elektroneneinfangzuständen und Lochzuständen in Kristallen her. Grundlage hierbei ist das etablierte algebraische diagrammatische Konstruktionsschema (ADC) zur Approximation der Selbstenergie für die Bestimmung der Vielteilchen-Green's-Funktion mittels der Dyson-Gleichung. Die volle Translationssymmetrie des Problems wird hierbei beachtet und die Lokalität elektronischer Korrelationen ausgenutzt. Das resultierende Schema wird Kristallorbital-ADC (CO-ADC) genannt. Ich berechne damit die Quasiteilchenbandstruktur einer Fluorwasserstoffkette und eines Lithiumfluoridkristalls. In beiden Fällen erhalte ich eine sehr gute Übereinstimmung zwischen meinen Resultaten und den Ergebnissen aus anderen Methoden. / In this dissertation, theories for the ab initio description of the states of perfect semiconducting and insulating crystals are derived and applied. Electron correlations are treated thoroughly based on the Hartree-Fock approximation formulated in terms of Wannier orbitals. In part I of the treatise, I study the ground state of hydrogen-bonded hydrogen fluoride and hydrogen chloride zig-zag chains. I analyse the long-range contributions of electron correlations. Thereby, I employ basis set extrapolation techniques, which have originally been developed for small molecules, to also obtain highly accurate binding energies of crystals. In part II of the thesis, I devise an ab initio description of the electron attachment and electron removal states of crystals using methods of quantum field theory. I harness the well-established algebraic diagrammatic construction scheme (ADC) to approximate the self-energy, used in conjunction with the Dyson equation, to determine the many-particle Green's function for crystals. Thereby, the translational symmetry of the problem and the locality of electron correlations are fully exploited. The resulting scheme is termed crystal orbital ADC (CO-ADC). It is applied to obtain the quasiparticle band structure of a hydrogen fluoride chain and a lithium fluoride crystal. In both cases, a very good agreement of my results to those determined with other methods is observed.

In dieser Dissertation werden ab initio Theorien zur Beschreibung der Zustände von perfekten halbleitenden und nichtleitenden Kristallen, unter Berücksichtigung elektronischer Korrelationen, abgeleitet und angewandt. Als Ausgangsbasis dient hierzu die Hartree-Fock Approximation in Verbindung mit Wannier-Orbitalen. Darauf aufbauend studiere ich zunächst in Teil I der Abhandlung den Grundzustand der wasserstoffbrückengebundenen Fluorwasserstoff und Chlorwasserstoff zick-zack Ketten und analysiere die langreichweitigen Korrelationsbeiträge. Dabei mache ich die Basissatzextrapolationstechniken, die für kleine Moleküle entwickelt wurden, zur Berechnung von hochgenauen Bindungsenergien von Kristallen nutzbar. In Teil II der Arbeit leite ich zunächst eine quantenfeldtheoretische ab initio Beschreibung von Elektroneneinfangzuständen und Lochzuständen in Kristallen her. Grundlage hierbei ist das etablierte algebraische diagrammatische Konstruktionsschema (ADC) zur Approximation der Selbstenergie für die Bestimmung der Vielteilchen-Green's-Funktion mittels der Dyson-Gleichung. Die volle Translationssymmetrie des Problems wird hierbei beachtet und die Lokalität elektronischer Korrelationen ausgenutzt. Das resultierende Schema wird Kristallorbital-ADC (CO-ADC) genannt. Ich berechne damit die Quasiteilchenbandstruktur einer Fluorwasserstoffkette und eines Lithiumfluoridkristalls. In beiden Fällen erhalte ich eine sehr gute Übereinstimmung zwischen meinen Resultaten und den Ergebnissen aus anderen Methoden. / In this dissertation, theories for the ab initio description of the states of perfect semiconducting and insulating crystals are derived and applied. Electron correlations are treated thoroughly based on the Hartree-Fock approximation formulated in terms of Wannier orbitals. In part I of the treatise, I study the ground state of hydrogen-bonded hydrogen fluoride and hydrogen chloride zig-zag chains. I analyse the long-range contributions of electron correlations. Thereby, I employ basis set extrapolation techniques, which have originally been developed for small molecules, to also obtain highly accurate binding energies of crystals. In part II of the thesis, I devise an ab initio description of the electron attachment and electron removal states of crystals using methods of quantum field theory. I harness the well-established algebraic diagrammatic construction scheme (ADC) to approximate the self-energy, used in conjunction with the Dyson equation, to determine the many-particle Green's function for crystals. Thereby, the translational symmetry of the problem and the locality of electron correlations are fully exploited. The resulting scheme is termed crystal orbital ADC (CO-ADC). It is applied to obtain the quasiparticle band structure of a hydrogen fluoride chain and a lithium fluoride crystal. In both cases, a very good agreement of my results to those determined with other methods is observed.