The electronic structure of hydrogen and muonium at the bond-centered
site in diamond is investigated using ab initio cluster calculations.
Correlation effects are accounted for by a configuration interaction e
xpansion and by the local density approximation in the density functio
nal approach. The hyperfine and superhyperfine parameters for anomalou
s muonium are determined by averaging over the spread of the muon wave
function. Good agreement with experimental hyperfine parameters is fo
und.