Interatomic Potentials for Iron under Extreme Conditions

The physical properties of iron under extreme high-pressure and high-temperature conditions are crucial for understanding the internal structure and evolutionary processes of Earth and terrestrial planets. To characterize the dynamic behavior of iron under the extreme conditions inside super-Earths, we combine first-principles molecular dynamics simulations with experimentally measured high-pressure melting curves to construct an embedded-atom potential applicable across ultra-high pressures and temperatures. This potential is fitted to multiple properties of the body-centered cubic (BCC), hexagonal close-packed (HCP), and liquid phases over 400 GPa to 1 TPa and 6000 to 10000 K, including the elastic constants of the solid phases, the radial distribution functions of the liquid, and experimentally determined melting data. We systematically validate the potential across different pressure-temperature conditions and found that it accurately reproduces the pressure and temperature dependence of solid elastic constants, and matches liquid radial distribution functions at three representative pressure-temperature conditions. Moreover, it predicts melting curves that lie within experimental uncertainties and agree well with previous first-principles simulations. Thermodynamic calculations based on this potential further show that the HCP phase remains thermodynamically stable between 400 GPa and 1 TPa, while the BCC phase is metastable. This potential provides a reliable atomistic tool for large-scale simulations of nucleation, crystallization, and solid-liquid coexistence in the cores of super-Earths. Moreover, the potential and associated dataset lay the groundwork for future extensions to multicomponent Fe alloys and their properties under ultra-high-pressure conditions.