It is shown that the inclusion of both electron-hole correlations and local-field effects in the intervalley matrix elements for point-charge impurities is indispensable for the binding energies. For a substitutional donor these many-electron effects reduce the binding energy by as much as 35\%, thus significantly strengthening the shallow character and introducing very good agreement with experiment (example Si:P). Combination of the authors' quantitative calculations with recent muon-decay experiments allows for a prediction of the interstitial muonium site in Si.
 

Physical Review B, 30 2266-9, 1984.