Self-interstitials in V and Mo

We report an extensive ab initio study of self-interstitials in V and Mo. Contrary to the widely accepted picture, the $〈111〉$ dumbbell is found to be the most stable structure. The activated state for migration is the crowdion configuration, with an extremely low barrier $(\ensuremath{\sim}0.01\mathrm{eV}),$ suggesting $1d$ (one-dimensional) diffusion at low temperatures and $3d$ diffusion at high temperature. In the case of Mo, the energy landscape between the $〈111〉$ and $〈110〉$ dumbbells is very shallow. Predicted migration energies and self-interstitial structures are consistent with experiment.