  The magnitude of finite-size effects and Coulomb interactions in quantum
Monte Carlo simulations of van der Waals interactions between weakly bonded
benzene molecules are investigated. To that extent, two trial wave functions of
the Slater-Jastrow and Backflow-Slater-Jastrow types are employed to calculate
the energy-volume equation of state. We assess the impact of the backflow
coordinate transformation on the non-local correlation energy.
  We found that the effect of finite-size errors in quantum Monte Carlo
calculations on energy differences is particularly large and may even be more
important than the employed trial wave function. Beside the cohesive energy,
the singlet excitonic energy gap and the energy gap renormalization of
crystalline benzene at different densities are computed.
