Coupling experiment and theory to push the state-of-the-art in X-ray spectroscopy.

X-ray spectroscopy plays a pivotal role in understanding the geometric and electronic structures of countless molecules and materials, from homogeneous and heterogeneous catalysts to biological active sites. The element-selectivity of X-ray spectroscopy allows for phenomena at specific photoabsorbers to be investigated. Since the early 2000s, experimental sophistication has progressed, with increasing applications of X-ray emission spectroscopy and two-dimensional photon-in-photon-out spectroscopies, such as resonant inelastic X-ray scattering. Although advanced X-ray spectroscopic methods increase selectivity and information content, the spectra obtained present major challenges for both qualitative and quantitative interpretation. To maximize the insight gained from X-ray spectroscopy, close coupling of experiment and theory is essential. Herein, we present the theoretical and experimental aspects of X-ray spectroscopy, with an emphasis on molecular systems and how an integrated approach with a solid foundation in molecular electronic structure theory enables new modes of inquiry into (bio)chemical catalysis.