  The iron-sulfur system is important for planetary interiors and is intensely
studied, particularly for better understanding of the cores of Mars and Earth.
Yet, there is a paradox about high-pressure stability of FeS: ab initio global
optimization (at DFT level) predicts a Pmmn phase (with a distorted rocksalt
structure) to be stable at pressures above ~120 GPa, which has not yet been
observed in the experiments that instead revealed a CsCl-type phase which,
according to density functional calculations, should not be stable. Using
quasiharmonic free energy calculations and the dynamical mean field theory, we
show that this apparent discrepancy is removed by proper account of electron
correlations and entropic effects.
