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X-ray spectroscopic techniques are of vast interest in different areas of science, such as physics, biology and, of course, chemistry. These techniques study the interaction between x-rays and matter, which is different depending on the material, and hence ideal for chemical characterisation. Indeed, from looking at x-ray spectra, one might be able to determine what chemical species is under study. However, the x-ray spectra hide much more information for chemists. By studying how the systems behave when interacting with x-rays, they can obtain information about the electronic structure of the material, that is, information about the electrons in it, such as their energy or their position in space. From the understanding of the electronic structure, chemists can predict different properties of the system. Because of their huge potential for research of new materials and drug design, x-ray techniques have been largely developed in the last years. This development has led to the construction of modern synchrotrons and x-ray free electron laser (XFEL) facilities, which make it possible to perform experiments so sophisticated that were barely imaginable a few years ago. As these experiments become more and more complex, so does their interpretation. In fact, the experimental spectra obtained nowadays at these installations are of little or no use without the corresponding theoretically calculated spectra that enable their interpretation. Therefore, the advances seen on the experimental side need to go together with advances on the theoretical one to be of any use for the scientific community. It is thus paramount to develop new and accurate methods that can keep up with the experimental progress. It is precisely the intention of this thesis to contribute to the progress of x-ray theory by developing new coupled-cluster methods, considered to be among the most accurate in quantum chemistry. In this regard, this thesis presents a new method to simulate different x-ray spectroscopic techniques named frozen-core core-valence separation equation-of-motion coupled-cluster. Among the various x-ray techniques, this work focuses primarily on K-edge and L-edge near-edge x-ray absorption fine structure (NEXAFS) and x-ray photo-electron spectroscopy (XPS), and their time-resolved (TR) variants. |
