MOMENTUM DISTRIBUTION OF LIQUID-HELIUM

We have obtained the one-body density matrix and the momentum distribu tion n(p) of liquid He-4 at T = 0 K from diffusion Monte Carlo (DMC) s imulations, using trial functions optimized via the Euler Monte Carlo (EMC) method. We find a condensate fraction smaller than in previous c alculations. Though we do not explicitly include long-range correlatio ns in our calculations, we obtain a momentum distribution at long wave length which is compatible with the presence of long-range correlation s in the exact wave function. We have also studied He-3, using fixed-n ode DMC, with nodes and trial functions provided by the EMC. In partic ular, we analyze the momentum distribution n(p) with respect to the di scontinuity Z as well as the singular behavior at the Fermi surface. W e also show that an approximate factorization of the one-body density matrix rho(r) similar or equal to rho(0)(r)rho(B)(r) holds, with rho(0 )(r) and rho(B)(r), respectively, the density matrix of the ideal Ferm i gas and the density matrix of a Bose He-3.