Density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials: Application to fcc-Pt and fcc-Au

I present density functional perturbation theory for lattice dynamics with the fully relativistic ultrasoft pseudopotentials (PPs) introduced recently for dealing with spin-orbit effects and use it to calculate the phonon dispersions of fcc-Pt and fcc-Au. The results are compared with the dispersions obtained by scalar relativistic PPs and by inelastic neutron scattering. It is found that, on the phonon spectrum of fcc-Au, spin-orbit effects are small, essentially within the numerical accuracy. In fcc-Pt, these effects, although still small, improve the agreement between theory and experiment close to the Kohn anomaly of the T-1 branch along the Sigma line.