Getting to the core of valence excitations from first principles

Excited-state processes in molecules, materials, and at interfaces can reveal intricate details of energy transfer between electronic and nuclear degrees of freedom: photo-induced chemistry, charge transfer, nonthermal melting, etc. Accurate theoretical methods can hypothesize on the ultrafast evolution of these excited states and advanced pump-probe characterization can reveal spectral signatures of the same, however, combining prediction and interpretation of ultrafast phenomena works best when we can simulate both pump and probe to directly connect experiment and theory. To this end, we focus on simulating core-level spectroscopy of ground and excited valence states from first principles. Valence excited states can be modeled in different ways, depending on the context, but here we focus on two approaches based on density functional theory (DFT): constrained-DFT and real-time time-dependent (TD) DFT. Core-excited states are generally modeled with Delta-SCF or linear-response TDDFT, and we outline our approach to combine valence and core excited states for direct interpretation of ultrafast X-ray and XUV probes. An abundance of local physical and chemical detail on valence excitations is evident through probing core-excitations, which we explore, atom by atom, in a predictive fashion.