Invited Lecture
Recent advances in theoretical calculations of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics

We review recent progress in theoretical simulations of lattice defects and lattice-vacancy diffusion coefficients based on ab initio electronic structure theory. In particular, we revisit the color-diffusion (CD) algorithm in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose substantially more efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting [1]. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate kNE increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. At the same time, equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study. Moreover, we demonstrate the applicability of the CD algorithm in simulations of Ti monovacancy jump frequencies in a compound, NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K.

[1] D. G. Sangiovanni, O. Hellman, B. Alling, and I. A. Abrikosov, Phys. Rev. B 93, 094305 (2016).