Real-time approaches are becoming increasingly important in understanding various photon spectroscopies ranging from linear and non-linear optical response to x-ray absorption spectra (XAS). In this talk I will discuss prospects for improved accuracy and for generalizing current techniques for molecular and condensed systems to high-fields and x-ray energies.

First I discuss extensions of our real-time, time-dependent density functional theory (RT-TDDFT) approach for calculations of frequency-dependent linear and non-linear optical response. This approach is based on the SIESTA calculations of the time-dependent Hamiltonian and a time-reversible Crank-Nicholson time-evolution operator. Second, I discuss an alternative approach based on local time-dependent Green's functions for obtaining core-level x-ray response using the SIESTA Hamiltonian. Third, I discuss techniques for improved calculations of core-level x-ray response using the GW/Bethe-Salpeter Equation approach and a quasi-boson model of electronic excitations.