  {\it Ab initio} calculations of relativistic electron energy loss spectra
(REELS) are carried out using a generalization of the real-space Green's
function code FEFF8 which is applicable to general aperiodic materials. Our
approach incorporates relativistic effects in terms of the cross-section tensor
within the dipole selection rule. In particular the approach explains
relativistic corrections to the magic angle in polarized EELS experiments. Our
generalization includes instrumental effects, such as the integral of the cross
section over the impulse transfer dependence, to account for a finite detector
aperture and electron beam width. The approach is illustrated with an
application to the graphite C K edge.
