Spectroscopy, whether photonic or electronic, constitutes an essential diagnostic tool for characterizing materials properties, ranging from structural information to electronic and excited state properties. Electronic excitations are probed by experimental techniques such as optical absorption, EELS and photoemission (direct or inverse). From the theory point of view, excitations and excited state properties are out of the reach of density-functional theory (DFT), which is a ground-state theory. In the last twenty years other ab-initio theories and frameworks, which are able to describe electronic excitations and spectroscopy, have become more and more used: time-dependent density-functional theory (TDDFT) and many-body perturbation theory (MBPT) or Green's function theory (GW approximation and Bethe-Salpeter equation BSE). In fact, computational solutions and codes have been developed in order to implement these theories and to provide tools to calculate excited state properties.The present school focuses on these points, covering theoretical, practical, and also numerical aspects of TDDFT and MBPT, and codes implementing them (ABINIT, DP, EXC). On the basis our previous experience with the organisation of schools (see Spectroscopy Lectures and ABINIT Summer School), the presentation of the theory will be followed by practical classes and hands-on tutorials. At the end of the school, students will have sufficient working knowledge to pursue their projects at their home institution. The participants are expected to have a fair knowledge of DFT prior to the school (see Ref. 1) and to be familiar with one plane-wave pseudopotential based software. Indeed, although at the beginning of the school, we will make sure that the DFT level of all participants is enough in this respect and provide the needed complementary information and training, the purpose of the school is to go beyond DFT, with hands-on exercices based on plane-wave implementations. So, in the application to the school, the experience with DFT and plane-wave based software(s) should be described. To be specific, it should be mentioned in the "CV and Motivation" box (section Short CV and research experience) obtained by clicking on "Apply" then "Application" keywords of the menu.
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