1. Applied Materials Physics, Department of Materials Science and Engineering, The Royal Institute of Technology, SE-100 44 Stockholm, Sweden
2. Condensed Matter Theory Group, Stockholm Center for Physics, Astronomy and Biotechnology, Department of Physics, The Royal Institute of Technology, SE-106 91 Stockholm, Sweden
3. Condensed Matter Theory Group, Department of Physics, Uppsala University, Box 530, SE-751 21, Uppsala, Sweden

Correspondence to: Anatoly B. Belonoshko^1,2 Correspondence and requests for materials should be addressed to A.B. (Email: anatoly@fysik.uu.se).

Iron is thought to be the main constituent of the Earth's core¹, and considerable efforts^{2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14} have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves¹⁵. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.–b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon^{16, 17, 18}, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.