  Diffusion Monte Carlo (DMC) simulations for fermions are becoming the
standard to provide high quality reference data in systems that are too large
to be investigated via quantum chemical approaches. DMC with the fixed-node
approximation relies on modifications of the Green function to avoid
singularities near the nodal surface of the trial wavefunction. We show that
these modifications affect the DMC energies in a way that is not
size-consistent, resulting in large time-step errors. Building on the
modifications of Umrigar {\em et al.} and of DePasquale {\em et al.} we propose
a simple Green function modification that restores size-consistency to large
values of time-step; substantially reducing the time-step errors. The new
algorithm also yields remarkable speedups of up to two orders of magnitude in
the calculation of molecule-molecule binding energies and crystal cohesive
energies, thus extending the horizons of what is possible with DMC.
