We provide two examples to illustrate how electronic structure calculations contribute to our understanding of vacancies and their role in determining material properties. Diffusion and elctromigration in aluminium are known to depend strongly on vacancies. Electronic structure calculations show that the vacancy-impurity interaction oscillates with distance, and this leads to an explanation for both the increased elctromigration resistance and the slow impurity diffusion for copper in aluminium. Calculations of vacancies in plutonium have been used in conjunction with positron annihilation lifetime measurements to identify the presence of helium-filled vacanies. Helium stabilization of vacancies can provide the precursors for subsequent vacancy-related changes in materials properties
