Positive-projection Monte Carlo simulation: A new variational approach to strongly interacting fermion systems

We show that very accurate ground-state energies and correlation functions can be obtained from an approximate, variational form of the widely used auxiliary-field simulation method. It is derived by recasting this method as a diffusion problem, and does not exhibit the poor statistical behavior due to vanishing normalization or ``sign'' common to exact fermion simulations. We give illustrative results for two-dimensional Hubbard models.