  The fixed node diffusion Monte Carlo (DMC) method has attracted interest in
recent years as a way to calculate properties of solid materials with high
accuracy. However, the framework for the calculation of properties such as
total energies, atomization energies, and excited state energies is not yet
fully established. Several outstanding questions remain as to the effect of
pseudopotentials, the magnitude of the fixed node error, and the size of
supercell finite size effects. Here, we consider in detail the semiconductors
ZnSe and ZnO and carry out systematic studies to assess the magnitude of the
energy differences arising from controlled and uncontrolled approximations in
DMC. The former include time step errors and supercell finite size effects for
ground and optically excited states, and the latter include pseudopotentials,
the pseudopotential localization approximation, and the fixed node
approximation. We find that for these compounds, the errors can be controlled
to good precision using modern computational resources, and that quantum Monte
Carlo calculations using Dirac-Fock pseudopotentials can offer good estimates
of both cohesive energy and the gap of these systems. We do however observe
differences in calculated optical gaps that arise when different
pseudopotentials are used.
