{\it Ab initio} calculations of optical spectra

We present a real space approach for {\it ab initio} calculations of the optical constants of materials. The approach is based on a generalization of the {\it ab initio} Green's function formalism implemented in the FEFF8 code to include valence and conduction band spectra. This all-electron approach, which is the real-space analog of the KKR method, includes self-consistent potentials, relativistic effects, inelastic losses, self-energy and screened core-hole effects, and multiple-scattering to all orders. Our procedure is based on calculations of the imaginary part of the dielectric function $\epsilon_2$, from which various optical constants can be derived using analytical properties and Kramers-Kronig transforms. Results for several materials are presented and compared with experiment.