  Motivated by the disagreement between recent diffusion Monte Carlo
calculations and experiments on the phase transition pressure between the
ambient and beta-Sn phases of silicon, we present a study of the HCP to BCC
phase transition in beryllium. This lighter element provides an oppor- tunity
for directly testing many of the approximations required for calculations on
silicon and may suggest a path towards increasing the practical accuracy of
diffusion Monte Carlo calculations of solids in general. We demonstrate that
the single largest approximation in these calculations is the pseudopotential
approximation. After removing this we find excellent agreement with experiment
for the ambient HCP phase and results similar to careful calculations using
density functional theory for the phase transition pressure.
