  We study the static screening in a Hubbard-like model using fixed-node
diffusion Monte Carlo. We find that the random phase approximation is
surprisingly accurate even for metallic systems close to the Mott transition.
As a specific application we discuss the implications of the efficient
screening for the superconductivity in the doped Fullerenes. In the Monte Carlo
calculations we use trial functions with two Gutzwiller-type parameters. To
deal with such trial functions, we introduce a method for efficiently
optimizing the Gutzwiller parameters, both in variational and in fixed-node
diffusion Monte Carlo.
