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FeO is a crucial phase of the Earth's core, and its thermodynamic propertiesare essential to developing more accurate core models. It is also a notoriouscorrelated insulator in the NaCl-type (B1) phase at ambient conditions. Itundergoes two polymorphic transitions at 300 K before it becomes metallic inthe NiAs-type (B8) structure at $\sim$100 GPa. Although its phase diagram isnot fully mapped, it is well established that the B8 phase transforms to theCsCl-type (B2) phase at core pressures and temperatures. Here we report asuccessful \textit{ab initio} calculation of the B8$\leftrightarrow$B2 phaseboundary in FeO at Earth's core pressures. We show that fully anharmonic freeenergies computed with the PBE-GGA + Mermin functional reproduce theexperimental phase boundary within uncertainties at $P > 240$ GPa, includingthe largely negative Clapeyron slope of $-52 \pm 5$ MPa/K. This study validatesthe applicability of a standard DFT functional to FeO under Earth's coreconditions and demonstrates the theoretical framework that enables complexpredictive studies of this region.

PBE-GGA Predicts the B8$\leftrightarrow$B2 Phase Boundary of FeO at Earth's Core Conditions