We have obtained the one--body density matrix and the momentum distribution
n(p) of liquid 4He at T=3D0oK from Diffusion Monte Carlo (DMC)
simulations, using trial functions optimized via the Euler Monte Carlo (EMC)
method. We find a condensate fraction smaller than in previous calculations.
Though we do not explicitly include long--range correlations in our
calculations, we get a momentum distribution at long wavelength which is
compatible with the presence of long--range correlations in the exact wave
function. We have also studied 3He, using fixed--node DMC, with nodes and
trial functions provided by the EMC. In particular, we analyze the momentum
distribution n(p) with respect to the discontinuity Z as well as the
singular behavior, at the Fermi surface. We also show that an approximate
factorization of the one-body density matrix ρ(r)≃ρ0(r)ρB(r)
holds, with ρ0(r) and ρB(r) respectively the density matrix of the
ideal Fermi gas and the density matrix of a Bose 3He.Comment: 10 pages, REVTeX, 12 figure