Excited states in complex molecular systems are in general multiscale, in the sense that intrinsic quantum-mechanical properties of the basic units, i.e. isolated molecules, and the local and global morphology of the large-scale molecular system are intertwined. At this scale, the use of GW-BSE-in-DFT embedding as sketched in the previous section can be computationally extremely demanding, if not prohibitive. Similar notions also apply to other quantum-quantum embedding strategies .