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.
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