Anna Krylov of the University of Southern California is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to extend the theoretical spectroscopy modeling toolkit by developing novel electronic structure methods in the core and valence domains with particular emphasis on free-electron states. Using light to interrogate matter is the basis of spectroscopy, which provides the most powerful set of tools for unraveling mechanisms of chemical reactions, structures and intrinsic properties of materials and biological objects. Using high-energy (X-rays) and high-intensity radiation opens new exciting opportunities, which motivate the worldwide development of multi-billion-dollar facilities for advanced light sources. Recent advances in beam quality in these facilities greatly expanded possible applications of X-rays, giving rise to a proliferation of techniques including those operating in time-resolved and non-linear regimes. These novel techniques promise to greatly expand our ability to interrogate molecular structure and dynamics, but their full potential can only be realized when experiment is augmented by accurate theoretical tools for modeling these phenomena. Despite significant efforts, the theory is still lagging behind the experimental capabilities, creating a bottleneck for maximizing the scientific impact of multi-billion advanced light source facilities. One of the challenges is that many techniques in