1 K. P. Driver driver@mps.ohio-state.edu, \\
1 J. W. Wilkins wilkins@mps.ohio-state.edu \\

Quantum Monte Carlo equations of state of $\alpha$- and $\beta$-magnesium silicate

1 Physics Dept., Ohio State University, 1040 Physics Research Building
  191 West Woodruff Avenue, Columbus, OH 43210-1117, United States


The 410 km seismic discontinuity in Earth's mantle is ascribed to the
$\alpha$ to $\beta$-(Mg,Fe)$_2$SiO$_4$ phase
transformation. Considering Mg-endmembers, density functional theory
(DFT) predictions within LDA and GGA disagree on the phase boundary by
50\% [1]. Quantum Monte Carlo (QMC) offers a route to avoid the
approximation of the exchange-correlation potential in DFT and provide
a benchmark for the phase boundary, elastic moduli, and thermodynamic
properties.  Zero point and thermal contributions are included by
using DFT linear response within the quasiharmonic
approximation. Preliminary results indicate the QMC phase relations
and bulk moduli are in reasonable agreement with experiment.

Computations were performed at NERSC, OSC, and CCNI.
Financial support provided by the DOE (DE-FG02-99ER45795).

[1]Y. Yu, Z. Wu., R. M. Wentzcovitch, Earth Planet. Sci. Lett. 273, 115 (2008).