Fermi surface nesting and phonon instabilities in simple cubic calcium

Phonon instabilities and Fermi surface nesting are studied in the high-pressure simple cubic phase of calcium by means of ab initio calculations. We have focused on nesting along Γ X, which could be responsible for some of the anomalies observed in the phonon spectrum. Phonon frequencies calculated with the density functional perturbation theory are imaginary at several Brillouin-zone points (e.g. at M). However, including anharmonic contributions to the potential might be crucial to stabilize simple cubic calcium, as solving the Schrödinger equation associated to the transversal unstable mode at M gives a positive frequency.