Electronic excitations, caused for example by irradiation with electrons, light or modern photon sources (synchrotron, ultra-fast lasers), are key quantities for the study of materials, ranging from solids to atoms, from surface to nanoscale systems. An accurate understanding and prediction of the interaction of radiation and matter can even favor remarkable technological breakthroughs, like new functionalities for bulk systems (e.g. in optoelectronics) or, in the long term, in biological applications ("bottom-up" assembling or molecular machines1).