  The size of the population of random walkers required to obtain converged
estimates in DMC increases dramatically with system size. We illustrate this by
comparing ground state energies of small clusters of parahydrogen (up to 48
molecules) computed by Diffusion Monte Carlo (DMC) and Path Integral Ground
State (PIGS) techniques. We contend that the bias associated to a finite
population of walkers is the most likely cause of quantitative numerical
discrepancies between PIGS and DMC energy estimates reported in the literature,
for this few-body Bose system. We discuss the viability of DMC as a
general-purpose ground state technique, and argue that PIGS, and even finite
temperature methods, enjoy more favorable scaling, and are therefore a superior
option for systems of large size.
