  We present a way to include non local potentials in the standard Diffusion
Monte Carlo method without using the locality approximation. We define a
stochastic projection based on a fixed node effective Hamiltonian, whose lowest
energy is an upper bound of the true ground state energy, even in the presence
of non local operators in the Hamiltonian. The variational property of the
resulting algorithm provides a stable diffusion process, even in the case of
divergent non local potentials, like the hard-core pseudopotentials. It turns
out that the modification required to improve the standard Diffusion Monte
Carlo algorithm is simple.
