  We introduce a method that combines the power of both the lattice Green
function Monte Carlo (LGFMC) with the auxiliary field techniques (AFQMC), and
allows us to compute exact ground state properties of the Hubbard model for U<~
4t on finite clusters.
  Thanks to LGFMC one obtains unbiased zero temperature results, not affected
by the so called Trotter approximation of the imaginary time propagator exp(- H
t). On the other hand the AFQMC formalism yields a remarkably fast convergence
in t before the fermion sign problem becomes prohibitive. As a first
application we report ground state energies in the Hubbard model at U/t=4 with
up to one hundred sites.
