Pressure-induced4foccupancy enhancement in the rare-earth metals

Calculations of $4f$ excitation energies as functions of the Wigner-Seitz radius for Ce, Sm, Er, and Yb indicate that the occupancy of the $4f$ shell increases at elevated pressures in the rare-earth metals. For each metal investigated we find a 3+ → 2+ transition in the range of a few Mbar. The Yb results also feature a 2+ → 3+ valence change in accord with recent experiments, while for Ce there is no evidence for a trivalent ($4f^1$) → tetravalent ($4f^0$) transition at any pressure.