  We employ the density functional theory to study a structural transition of
MnO from B1 (rocksalt) to B8 (NiAs) structures that was observed experimentally
at pressures around 100 GPa. We utilize all-electron description as well as
norm-conserving pseudopotentials and demonstrate that these two approaches can
significantly differ in quantitative predictions. We explicitly show that even
small-core pseudopotentials exhibit transferability inaccuracies for quantities
sensitive to the energy differences between high- and low-spin polarizations of
valence electrons.
