Microscopic calculation of the phonon-roton branch in superfluid $^4$He
  Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk liquid $^4$He 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. 
J. Boronat and J. Casulleras (Univ. Politecnica de Catalunya,   Barcelona, Spain)