  We have studied the diamond -> beta-tin phase transition in Si using
diffusion quantum Monte Carlo (DMC) methods. Slater-Jastrow-backflow trial wave
functions give lower DMC energies than Slater-Jastrow ones, and backflow
slightly favors the beta-tin phase with respect to the diamond phase. We have
investigated the changes in the equation of state that result from the use of
different pseudopotentials, the inclusion of either zero-point motion or
finite-temperature vibrations, and the application of corrections for
finite-size effects. Our tests indicate that the choice of pseudopotential can
significantly affect the equation of state. Using a Dirac-Fock pseudopotential
leads to an overestimation of the transition pressure but an empirical
pseudopotential designed for use in correlated calculations gives a transition
pressure in quite good agreement with experiment.
