The standard approach to explaining band structure is to assume that the electrons in a solid move in a potential from the ions, which is periodic leading to Blochs Theorem and the formation of band structure. But I am a bit confused at this point. Is the approach only valid for the valence electrons in the solid? I.e. are the electrons assumed frozen out? It seems this is the case in many textbooks. If so, what justifies this approximation? In an earlier post I already touched upon the Born-Oppenheimer approximation, but this is about decoupling the ionic wavefunctions based on the big difference in their masses and not that of the core electrons. On the other hand, if the approach were valid for all the electrons in the solid it would make sense, since the inert core electrons would then be the ones that occupy the filled up bands and would then offer an explanation for explanation for why it is valid to separate the core- and valence electrons.