EULER MONTE-CARLO CALCULATIONS FOR LIQUID-HE-4 AND HE-3

We present an optimization procedure for Monte Carlo variational calcu lations and illustrate it by optimizing pair and triplet correlations in the ground-state wave functions of liquid He-4 and He-3. We use the optimized trial functions, with a partially optimized backflow for He -3, to perform extensive diffusion Monte Carlo simulations that yield accurate results for the equation of state and structural properties o f both liquids. The agreement with experiments, which is excellent for He-4, somewhat deteriorates in going to He-3, due to the use of the f ixed-node approximation. The remaining appreciable discrepancies betwe en the results of variational and diffusion Monte Carlo unambiguously point to the increasing importance of n-body correlations, with n>3, a s the density is increased.