MICROSCOPIC CALCULATION OF THE PHONON-ROTON BRANCH IN SUPERFLUID HE-4

Diffusion Monte Carlo results for the phonon-roton excitation branch i n bulk liquid He-4 at zero temperature are presented. The sign problem associated to the excited wave function has been dealt both with the fixed-node approximation and the released-node technique. The upper bo unds provided by the fixed-node approximation are shown to become exac t when using the released-node method. An excellent agreement with exp erimental data is achieved both at the equilibrium and near the freezi ng densities. The leading order correction beyond the Feynman-Cohen ex cited wave function shows the presence of three-body correlations in t he excitation operator.