Ab-initio phonon dispersions of fcc-Pb: effects of spin-orbit coupling

I present the ab-initio phonon dispersions of fcc-Pb calculated within the framework of density functional perturbation theory, with plane-waves and a fully relativistic ultrasoft pseudopotential which includes spin-orbit coupling effects. I find that, within the local density approximation, the theory gives phonon frequencies close to the experimental inelastic neutron scattering data. Many of the anomalies present in these dispersions are well reproduced by the fully relativistic pseudopotential theory and can be shown to appear only for small values of the smearing parameter that controls the sharpness of the Fermi surface.