  The self-interaction corrected density-functional calculations are performed
for Re impurities and their pairs in silicon. Rhenium ions form in the host
crystal not very tight pairs, with impurities separated by one Si atom or by a
distance close to two silicon bonds. Comparison of formation energies for
various pairs of substitutionals, interstitials, and mixed-site impurities
favours the last type. Electron transfer from the interstitial into the
substitutional impurity makes the both Re sites nonmagnetic, but the p-type and
the n-type co-doping revives magnetism again, the latter more efficiently.

Materials Science;Strongly Correlated Electrons