  Theoretical positron lifetime values have been calculated systematically for
most of the elements of the Periodic Table. Self-consistent and
non-self-consistent schemes have been used for the calculation of the
electronic structure in the solid, as well as different parameterizations for
the positron enhancement factor and correlation energy. The results obtained
have been studied and compared with experimental data, confirming the
theoretical trends. As it is known, positron lifetimes in bulk show a periodic
behaviour with atomic number. These calculations also confirm that monovacancy
lifetimes follow the same behaviour. From this fact a strong relation between
the atomic volume and the positron lifetime has been set. The effects of
enhancement factors used in calculations have been commented. Finally, we have
analysed the effects that f and d electrons cause in positron lifetimes.
