Ab initio calculation of formation and migration volumes for vacancies in Li and Na

The formation, migration, and activation volumes for monovacancies in Li and Na for zero and finite external pressure are calculated nb initio within the framework of the local-density approximation and the ab initio pseudopotential method. In both materials the activation volumes are smaller than half of the atomic volume. The approximations involved in the transition-state theory which is the basis for the calculations are discussed with special emphasis on the limitations of the theory at very high pressure. The results are compared with experimental data on self-diffusion.