Electron energy loss spectroscopy (EELS) probes electronic excitations of a solid on the atomic scale. The widespread availability of first-principles calculations has lead to an explosion of theoretical calculations of EELS spectra. Agreement between theory and experiment is generally reported to be good at the typical energy resolutions in commercial microscopes of 0.7-1.3 eV. However a brief survey of the X-ray absorption literature suggests that the anticipated introduction of monochromators, along with improvements in energy stability, and spectrometer resolution will unmask many more effects that cannot be predicted as precisely as they can be measured.

The shape and binding energy of a core excitation is determined by both the ground state electronic structure (initial state effects) and the reponse to the excited electron-hole (final state effects) (Fig. 1). Errors in the initial state, such as the systematic errors in band gaps (and hence band offsets) are inherent in the local density approximation eigenvalues used to simulate EELS spectra.