Diffusion Monte Carlo methods with a fixed number of walkers

In this paper we discuss various aspects of diffusion Monte Carlo methods u sing a fixed number of walkers. First, a rigorous proof of the divergence o f pure diffusion Monte Carlo (PDMC) methods (DMC without branching in which the weights are carried along trajectories) is given. Second, a bias-free Monte Carlo method combining DMC and PDMC approaches, and based on a minima l stochastic reconfiguration of the population, is discussed. Finally, some illustrative calculations for a system of coupled quantum rotators are pre sented.