Abstract. We discuss an efficient approach for calculations of optical to x-ray response based on a spectral function formalism which is implemented in an extension to the real-space Green's function code FEFF. This formalism avoids the need to compute wave-functions explicitly, and is applicable to arbitrary, aperiodic systems. Starting from the complex dielectric constant, the approach gives a number of linear optical constants, including the complex index of refraction the photoabsorption coefficient and energy-loss spectra over a very broad spectral range. These results provide a theoretical complement to standard tables and are useful in a variety of applications including ab initio calculations self-energy shifts, inelastic losses, mean-free paths, and stopping powers. For example, a many-pole self-energy model based on these results makes possible improved, parameter free calculations of various photon- and electron-spectra. The method is compared with experiment and with a pseudopotential-planewave Bethe-Salpeter Equation method.