Abstract
Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk liquid 4He 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 bounds provided by the fixed-node approximation are shown to become exact when using the released-node method. An excellent agreement with experimental data is achieved both at the equilibrium and near the freezing densities. The leading order correction beyond the Feynman-Cohen excited wave function shows the presence of three-body correlations in the excitation operator.