Real-time Approaches for Optical and X-ray Spectra

J. J. Rehr

Dept. of Physics, Univ. of Washington, Seattle, Wa 98195

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.