Real Space Multiple-Scattering Calculations of Optical Properties

We present a method for {\it ab initio} calculations of the dielectric function which is applicable for periodic and non-periodic materials alike. Our approach is a generalization to finite momentum transfer of that implemented in the real space multiple-scattering code FEFF8 and is the real-space analog of the KKR method. The approach includes self-consistent potentials, an energy dependent self-energy and screened core-hole effects. The method yields the dynamic structure factor $S(\vec q,\omega)$ over a broad spectrum from the optical to x-ray energies, and in the long-wavelength limit, yields various optical constants. Results for several materials are presented and compared with experiment.