  We report a new theoretical method for X-ray absorption spectroscopy (XAS) in
condensed matter which is based on the multi-channel multiple scattering theory
of Natoli et al. and the eigen-channel R-matrix method. While the highly
flexible real-space multiple scattering (RSMS) method guarantees a precise
description of the single-electron part of the problem, multiplet-like electron
correlation effects between the photo-electron and localized electrons can be
taken account for in a configuration interaction scheme. For the case where
correlation effects are limited to the absorber atom, a technique for the
solution of the equations is devised, which requires only little more
computation time than the normal RSMS method for XAS. The new method is
described and an application to XAS at the Ca $L_{2,3}$-edge in bulk Ca, CaO
and CaF$_2$ is presented.
