Electronic excited states of f-elements (lanthanides and actinides) in solids are involved in many advanced technologies ranging from solid-state lighting (SSL) to advanced nuclear energy systems (ANES). Their characteristics from the point of view of first-principles calculations and their computational demands are discussed in ...[+]

Electronic excited states of f-elements (lanthanides and actinides) in solids are involved in many advanced technologies ranging from solid-state lighting (SSL) to advanced nuclear energy systems (ANES). Their characteristics from the point of view of first-principles calculations and their computational demands are discussed in this presentation. Some wave function based applications (two-step CASSCF/CASPT2 + spin-orbit CI embedded-cluster calculations) are shown in materials like Ce-doped YAG, Yb-doped CsCaBr3, and U-doped Cs2NaYCl6, aimed at illustrating complexities of the computational problems and potentialities of first-principles calculations, and to introduce next future computational challenges implied by these types of manifolds of excited states[-]