Due to self-interaction errors, -electrons are not handled well by presently available density functionals. In particular, partially filled states are often incorrectly described, leading to large errors for Pr-Eu and Tb-Yb where the error increases in the middle (Gd is handled reasonably well, since 7 electrons occupy the majority shell). These errors are DFT and not VASP related. Particularly problematic is the description of the transition from an itinerant (band-like) behavior observed at the beginning of each period to localized states towards the end of the period. For the elements, this transition occurs already in La and Ce, whereas the transition sets in for Pu and Am for the elements. A routine way to cope with the inabilities of present DFT functionals to describe the localized electrons is to place the electrons in the core. Such potentials are available and described below. Furthermore, PAW potentials in which the states are treated as valence states are available, but these potentials are not expected to work reliable when the electrons are localized. Expertise using hybrid functionals or an LDA+U like treatment are not particularly large, but hybrid functionals should improve the description, if the electrons are localized, although the most likely fail of the electrons for band-like (itinerant) states.
In addition, special GGA potential are supplied for Ce-Lu, in which -electrons are kept frozen in the core (standard model for the treatment of localised electrons). The number of -electrons in the core equals the total number of valence electrons minus the formal valency. For instance: according to the periodic table Sm has a total of 8 valence electrons (6 electrons and 2 electrons). In most compounds Sm, however, adopts a valency of 3, hence 5 electrons are placed in the core, when the pseudopotential is generated (the corresponding potential can be found in the directory Sm_3). The formal valency is indicted by _n, where n is either 3 or 2. Ce is for instance a Ce potential for trivalent Ce (for tetravalent Ce the standard potential should be used).