  We have carried out quantum Monte Carlo (QMC) calculations of silicon crystal
focusing on the accuracy and systematic biases that affect electronic structure
characteristics. We show that 64 and 216 atom supercells provide an excellent
consistency for extrapolated energies per atom in the thermodynamic limit for
ground, excited, and ionized states. We have calculated the ground state
cohesion energy with both systematic and statistical errors below $\approx$
0.05 eV. The ground state exhibits a fixed-node error of only $\approx 1.3$% of
the correlation energy, suggesting an unusually high accuracy of the
corresponding single-reference trial wave function. We obtain a very good
agreement between optical and quasi-particle gaps that affirms the marginal
impact of excitonic effects. Our most accurate results for band gaps differ
from the experiments by about 0.2 eV. We assign this difference to a
combination of residual finite-size and fixed-node errors. We have estimated
the crystal Fermi level referenced to vacuum that enabled us to calculate the
edges of valence and conduction bands in agreement with experiments.
