  While Diffusion Monte Carlo (DMC) is in principle an exact stochastic method
for \textit{ab initio} electronic structure calculations, in practice the
fermionic sign problem necessitates the use of the fixed-node approximation and
trial wavefunctions with approximate nodes (or zeros) must be used. This
approximation introduces a variational error in the energy that potentially can
be tested and systematically improved. Here, we present a computational method
that produces trial wavefunctions with systematically improvable nodes for DMC
calculations of periodic solids. These trial wavefunctions are efficiently
generated with the configuration interaction using a perturbative selection
made iteratively (CIPSI) method. A simple protocol in which both exact and
approximate results for finite supercells are used to extrapolate to the
thermodynamic limit is introduced.
