How can a theoretician at SOLEIL be useful?

I will present some case studies where the tight interlinking between theory and experiment has been crucial for the investigation of electronic excitations in materials ranging from simple metals to transition metal oxides [1-4]. What is calculated and what is measured are often different: I will highlight the importance of bridging the gap between theory and experiment to uncover new physics. Finally, I will show how the idea that electronic correlation can be explained in terms of coupling of excitations promotes fruitful connections between different spectroscopies. This work has been done in collaboration with several members and collaborators of the Palaiseau Theoretical spectroscopy group.