The annihilation characteristics of positrons that were trapped at metal vacancies were calculated from first principles. The calculations were based upon various implementations of 2-component density-functional theory. A number of numerical methods were used to solve the resultant Kohn-Sham equations. It was shown that  ab initio  calculations provided reliable values for positron lifetimes in metals. The calculated ratios of the lifetimes for unrelaxed vacancies and the corresponding perfect bulk lattices were usually slightly too large, but a slight inwards relaxation of the vacancies could bring the ratios closer to the experimental values. The calculated lifetimes were quite independent of the shape approximations which were made in the calculations. Self-consistency of the electronic structure was found to be the more important factor. More conventional schemes for localized positron states produced almost the same results as did sophisticated 2-component calculations. A Brillouin-zone integration over the lowest-lying positron state was found to be necessary, in super-cell calculations of localized positron states, in order to obtain rapid convergence of the results with respect to the size of the super-cell. The initial calculations which were made here indicated that Brillouin-zone integration was even more important for the more delocalized positron states at vacancy-type defects in semiconductors.

T.Korhonen, M.J.Puska, R.M.Nieminen: Physical Review B, 1996, 54[21], 15016-24