Ultrasoft pseudopotentials applied to magnetic Fe, Co, and Ni: From atoms to solids

We present a study of the accuracy, transferability, and plane-wave convergence properties of ultrasoft Vanderbilt-type pseudopotentials for Fe, Co, and Ni in the context of atomic, molecular, and solid calculations. Special attention has been given to the magnetic properties of these systems. To go beyond the local-spin-density-approximation, generalized gradient approximations for the exchange-correlation functional have been included. All calculations have been performed using a plane-wave basis set, and we show that ultrasoft pseudopotentials allow -- as expected -- for a considerably lower cutoff energy than standard soft norm-conserving pseudopotentials. Lattice properties show very good agreement with all-electron calculations and experiment, while larger discrepancies exist for magnetic structural energy differences (which however remain smaller than 2 mRy/atom). These differences can be traced back to the frozen core approximation which is implicitly assumed in the construction of the pseudopotentials. More accurate results for the magnetization energies of atomic configurations can be obtained by treating the $3p$ semicore states as valence states.