Ab initio phonon dispersions of Fe and Ni

We present the ab initio phonon dispersions of magnetic bcc Fe and fcc Ni. Our calculations are carried out in the framework of density functional perturbation theory (DFPT), using ultrasoft pseudopotentials, spin-polarized generalized gradient approximations, and nonlinear core corrections. The implementation of the above techniques within DFPT is discussed. We find that these approximations, together, provide phonon dispersions which are in good agreement with experiment, while the local spin density approximation systematically overestimates the experimental frequencies.