Nonlinear Spectroscopy of Core and Valence Excitations Using Short X-Ray Pulses: Simulation Challenges.

Measuring the nonlinear response of electrons and nuclei to attosecond broadband X-ray radiation has become possible by newly developed free electron lasers and high harmonic generation light sources. The design and interpretation of these novel experiments poses considerable computational challenges. In this chapter we survey the basic description of nonlinear X-ray spectroscopy signals and the electronic structure protocols which may be used for their simulation.