  The self-interaction corrected local spin density (SIC-LSD) formalism and the
standard GGA treatment of the exchange-correlation energy have been applied to
study the collapse of the magnetic moment of Fe impurities in MgO. The system
Mg_{1-x}Fe_xO is believed to be the second most abundant mineral in the Earth's
lower mantle.
  We confirm the experimentally found increase of the critical pressure upon
iron concentration. Our calculations using standard GGA for a fixed Fe
concentration show that different arrangements of Fe atoms can remarkably shift
the transition pressure of the high spin (HS) to low spin (LS) transition. This
could explain the experimentally found broad transition regions. Our results
indicate that the HS-LS transition in Mg$_{1-x}Fe_xO is first order. We find
that SIC-LSD fails to predict the divalent Fe configuration as the lowest
energy configuration and discuss possible reasons for it.
