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X-ray spectroscopy excites core orbitals, which are typically local, with element-specific energies, yet sensitive to the chemical environment. A new generation of light sources, such as the free-electron laser, allows the extension of numerous spectroscopies to the X-ray domain. The overall objective of this thesis is to explore these new spectroscopies by theory and provide tools for assignment and further understanding. We shall be working within a 4-component relativistic framework, not only to be able to address all elements of the periodic table, but also to take into account that relativistic effects are particularly pronounced in the core regions explored by X-ray spectroscopy. It also turns out that the formalism describing light-matter interaction is greatly simplified by working in such a framework.

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We report experimental differential cross sections (DCSs) for electron impact excitation of bands I to V of benzene at incident energies of 10, 12.5, 15, and 20 eV. They are compared to calculations using the Schwinger multichannel method while accounting for up to 437 open channels. For intermediate scattering angles, the calculations reveal that the most intense band (V) emerges from surprisingly similar contributions from all its underlying states (despite some preference for the dipole-allowed transitions). They further shed light on intricate multichannel couplings between the states of bands I to V and higher-lying Rydberg states. In turn, the measurements support a vibronic coupling mechanism for excitation of bands II and IV and also show an unexpected forward peak in the spin-forbidden transition accounting for band III. Overall, there is decent agreement between theory and experiment at intermediate angles and at lower energies and in terms of the relative DCSs of the five bands. Discrepancies between the present and previous experiment regarding bands IV and V draw attention to the need of additional experimental investigations. We also report measured DCSs for vibrational excitation of combined C–H stretching modes.

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In this work, we investigate the electronic structure of a particular class of carbon nanocones having a pentagonal tip and C5v symmetry. The ground-state nature of the wave function for these structures can be predicted by the recently proposed generalized Hückel rule that extends the original Hückel rule for annulenes to this class of carbon nanocones. In particular, the structures here considered can be classified as closed-shell or anionic/cationic closed-shells, depending on the geometric characteristics of the cone. The goal of this work is to assess the relationship between the electronic configuration of these carbon nanocones and their ability to gain or lose an electron as well as their adsorption capability. For this, the geometry of these structures in the neutral or ionic forms, as well as systems containing either one lithium or fluorine atom, was optimized at the DFT/B3LYP level. It was found that the electron affinity, ionization potential, and the Li or F adsorption energy present an intimate connection to the ground-state wave function character predicted by the generalized Hückel rule. In fact, a peculiar oscillatory energy behavior was discovered, in which the electron affinity, ionization energy, and adsorption energies oscillate with an increase in the nanocone size. The reasoning behind this is that if the anion is closed-shell, then the neutral nanocone will turn out to be a good electron acceptor, increasing the electron affinity and lithium adsorption energy. On the other hand, in the case of a closed-shell cation, this means that the neutral nanocone will easily lose an electron, leading to a smaller ionization potential and higher fluorine adsorption energy.

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Galvinoxyl, as one of the most extensively studied organic stable free radicals, exhibits a notable phase transition from a high-temperature (HT) phase with a ferromagnetic (FM) intermolecular interaction to a low-temperature (LT) phase with an antiferromagnetic (AFM) coupling at 85 K. Despite significant research efforts, the crystal structure of the AFM LT phase has remained elusive. This study successfully elucidates the crystal structure of the LT phase, which belongs to the P[1 with combining macron] space group. The crystal structure of the LT phase is found to consist of a distorted dimer, wherein the distortion arises from the formation of short intermolecular distances between anti-node carbons in the singly-occupied molecular orbital (SOMO). Starting from the structure of the LT phase, wave function calculations show that the AFM coupling 2J/kB varies significantly from −1069 K to −54 K due to a parallel shift of the molecular planes within the dimer.

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To expand the existing QUEST database of accurate vertical transition energies [\href{}{V\'eril et al.~\textit{WIREs Comput.~Mol.~Sci.} \textbf{2021}, \textit{11}, e1517}], we have modeled more than 100 electronic excited states of different natures (local, charge-transfer, Rydberg, singlet, and triplet) in a dozen of mono- and di-substituted benzenes, including aniline, benzonitrile, chlorobenzene, fluorobenzene, nitrobenzene, among others. To establish theoretical best estimates for these vertical excitation energies, we have employed advanced coupled-cluster methods including iterative triples (CC3 and CCSDT) and, when technically possible, iterative quadruples (CC4). These high-level computational approaches provide a robust foundation for benchmarking a series of popular wave function methods. The evaluated methods all include contributions from double excitations (ADC(2), CC2, CCSD, CIS(D), EOM-MP2, STEOM-CCSD), along with schemes that also incorporate perturbative or iterative triples (ADC(3), CCSDR(3), CCSD(T)(a)$^\star$, and CCSDT-3). This systematic exploration not only broadens the scope of the QUEST database but also facilitates a rigorous assessment of different theoretical approaches in the framework of a homologous chemical series, offering valuable insights into the accuracy and reliability of these methods in such cases. We found that both ADC(2.5) and CCSDT-3 can provide consistent estimates, whereas among less expensive methods SCS-CC2 is likely the most effective approach. Importantly, we show that some lower order methods may offer reasonable trends in the homologous series while providing quite large average errors, and \emph{vice versa}. Consequently, benchmarking the accuracy of a model based solely on absolute transition energies may not be meaningful for applications involving a series of similar compounds.

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Range separation Chemical concepts Ion Abiotic degradation Coupled cluster calculations Benchmarks AB-INITIO Diatomic molecules Xenon ALGORITHM Relativistic corrections Pesticides Metabolites Clustering Molecular modeling Environmental fate Partial least squares Rydberg states Spin-orbit interactions Parity violation A posteriori Localization Biodegradation AROMATIC-MOLECULES Configuration Interaction 3115ae 3115aj Electron correlation Aimantation Argon Wave functions BENZENE MOLECULE Single-core optimization Ground states 3115vj Atomic and molecular structure and dynamics Time-dependent density-functional theory Large systems X-ray spectroscopy Electron electric dipole moment Atomic processes Hyperfine structure États excités Excited states Quantum chemistry Atomic data Benzene Atomic and molecular collisions Pesticide Dispersion coefficients BIOMOLECULAR HOMOCHIRALITY Atrazine Quantum Chemistry 3315Fm Argile Analytic gradient Basis set requirements Molecular descriptors Azide Anion Configuration interaction Auto-énergie Atoms 3115ag Relativistic quantum mechanics 3115am Petascale CP violation Atom Beyond Standard Model New physics Dirac equation AB-INITIO CALCULATION Perturbation theory Atomic charges Parallel speedup Corrélation électronique 3470+e Ab initio calculation Basis sets Electron electric moment Carbon Nanotubes 3115bw A priori Localization Atomic charges chemical concepts maximum probability domain population Line formation Time reversal violation Density functional theory Valence bond Chimie quantique Configuration interactions QSAR Molecular properties Relativistic quantum chemistry Polarizabilities Acrolein Coupled cluster Quantum Monte Carlo 3115vn Dipole Mécanique quantique relativiste BSM physics CIPSI Atrazine-cations complexes Anderson mechanism Diffusion Monte Carlo Numerical calculations


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