Density matrix calculation of optical constants from optical to x-ray   frequencies
  We present a theory of linear optical constants based on a single-particle density matrix and implemented in an extension of the real-space multiple scattering code FEFF. This approach avoids the need to compute wave-functions explicitly, and yields efficient calculations for frequencies ranging from the IR to hard x-rays, and applicable to arbitrary aperiodic systems. Our approach is illustrated with calculations of optical properties and applications for several materials. 
M. P. Prange, J. J. Rehr, G. Rivas, J. J. Kas, John W. Lawson