Equation of state of hexagonal closed packed iron under Earth's core conditions from quantum Monte Carlo calculations

We applied quantum Monte Carlo techniques to compute the equation of state of hexagonal closed packed iron in the range of pressure relevant to Earth's core. We used an accurate iron pseudopotential with a frozen Ne core. Trial wave functions have been obtained from density-functional theory (DFT) plane-wave calculations and expanded in systematically improvable B splines. Tests with various exchange-correlation functionals showed that the B3LYP functional is the one that provided the best trial wave functions. Diffusion Monte Carlo calculations were carried out using simulation cells with up to 96 atoms (1536 electrons), with some attempts to use up to 150 atoms, and corrected for finite-size errors using the scheme of Chiesa [Phys. Rev. Lett. 97, 076404 (2006)] and Kwee [Phys. Rev. Lett. 100, 126404 (2008)]. The calculated equation of state agrees closely with the experiments of Mao [J. Geophys. Res. 95, 21737 (1990)] and those of Dewaele [Phys. Rev. Lett. 97, 215504 (2006)]. It also agrees with the DFT data of Soderlind [Phys. Rev. B 53, 14063 (1996)] and Alfe [Phys. Rev. B 61, 132 (2000)], and therefore, reinforces those previous calculations.