X-ray spectroscopic techniques are of vast interest in the different areas of naturals science. By studying how the x-rays interact with matter, one can obtain information about the properties of the system, such as the geometrical structure, the oxidation state of elements or their bonding environment.
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 the next generation of synchrotrons and x-ray electron laser (XFEL) facilities, capable of performing 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 in 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 profitable for the scientific community. It is therefore 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. Among these, near-edge x-ray absorption fine structure (NEXAFS) and x-ray photo-electron spectroscopy (XPS), and their time-resolved (TR) variants