Defect properties and diffusion in sodium using ab initio molecular dynamics

A Car–Parrinello-type linear-scaling scheme which uses the density as the basic variable, dispensing from orbitals, is used to simulate the finite-temperature behaviour of point defects in sodium. We briefly illustrate the method and show that it gives good results when applied to simple metals, using however only a fraction of the computational resources required by more conventional schemes. Results for the (fully anharmonic) free energies and for the structural properties of point defects are obtained, showing that the interstitial is more common than that previously thought of and it might even be the dominant defect at temperatures close to the melting point. Preliminary results on the relative contributions of the various defect mechanisms to sodium self-diffusion coefficient obtained via direct simulation are discussed, with special reference to the long-standing problem of self-diffusion in bcc metals.