  Classical turning surfaces of Kohn-Sham potentials, separating
classically-allowed regions (CARs) from classically-forbidden regions (CFRs),
provide a useful and rigorous approach to understanding many chemical
properties of molecules. Here we calculate such surfaces for several
paradigmatic solids. Our study of perfect crystals at equilibrium geometries
suggests that CFRs are absent in metals, rare in covalent semiconductors, but
common in ionic and molecular crystals. A CFR can appear at a monovacancy in a
metal. In all materials, CFRs appear or grow as the internuclear distances are
uniformly expanded. Calculations with several approximate density functionals
and codes confirm these behaviors. A classical picture of conduction suggests
that CARs should be connected in metals, and disconnected in wide-gap
insulators. This classical picture is confirmed in the limits of extreme
uniform compression of the internuclear distances, where all materials become
metals without CFRs, and extreme expansion, where all materials become
insulators with disconnected and widely-separated CARs around the atoms.
