  The lattice dynamics of hcp crystalline 4He is studied at zero temperature
and for two different densities (near and far from melting), using a
ground-state path-integral quantum Monte Carlo technique. The complete phonon
dispersion is obtained, with particular attention to the separation of optic
and acoustic branches and to the identification of transverse modes. Our study
also sheds light onto the residual coherence affecting quasiparticle
excitations in the intermediate momentum region, in-between the phonon and
nearly-free particle regimes.
