Implementation of ultrasoft pseudopotentials inab initiomolecular dynamics

A scheme for the construction of ultrasoft separable pseudopotentials recently proposed by Vanderbilt is tested in the context of Car-Parrinello ab initio molecular-dynamics calculations on atoms and molecules. For the case of oxygen, the transferability of the pseudopotential is demonstrated by comparing the calculated properties of molecular ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{3}$ with those obtained from conventional approaches. Converged results are obtained using plane-wave-basis cutoffs of only \ensuremath{\sim}25 Ry. Forces can be calculated efficiently, and a molecular-dynamics simulation of molecular vibration is demonstrated.