Comparative study of the PBE and SCAN functionals: The particular case of alkali metals.

The SCAN meta-generalized gradient approximation (GGA) functional is known to describe multiple properties of various materials with different types of bonds with greater accuracy, compared to the widely used PBE GGA functional. Yet, for alkali metals, SCAN shows worse agreement with experimental results than PBE despite using more information about the system. In the current study, this behavior for alkali metals is explained by identifying an inner semicore region which, within SCAN, contributes to an underbinding. The inner semicore push toward larger lattice constants is a general feature but is particularly important for very soft materials, such as the alkali metals, while for harder materials the valence region dominates.