Real Space Multiple Scattering Calculations of Relativistic Electron Energy Loss Spectra

{\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.