In this talk I will present theoretical developments of materials science and solid state physics, that are relevant for MAX Lab and ESS. The theoretical developments that I will present have focus on electron correlations, and how one may calculate the valence electron spectra and X-ray absorption spectra of archetype correlated electron systems. In this process the key features of dynamical mean field theory will be highlighted. A connection between theory of high energy excitations (electrons) and of low energy excitations (phonons and magnons) will be made, in a multi-scale approach and examples will be given for several classes of materials. These excitations are naturally probed by neutron scattering experiments and the implications of these results and the operation of ESS (European Spallation Source) will shortly be discussed.
