The electronic structure of hydrogen and muonium at the bond‐centered site (anomalous muonium) in silicon is investigated using ab initio cluster calculations. Correlation effects are accounted for by the configuration interaction method and by density functional calculations. A detailed investigation of the different functionals is presented. The calculations using configuration interaction and the different gradient corrected density functionals are in good agreement with each other. The effect of the zero point motion was included in the investigation.