Reducing the time-step errors in diffusion Monte Carlo

We modify the reweighting factor of the projector used in diffusion Monte Carlo to reduce the time-step error of the total energy. Furthermore, we present a reweighting scheme that has the desirable feature that it is exactly size-consistent, i.e., the energy of a system containing widely separated fragments is the same as the sum of the energies of the individual fragments. The practical utility of the latter improvement is that it reduces the time-step error of the binding energies of some weakly interacting systems.