In order to elucidate the thermal vibrations of hydrogen and normal muonium atoms trapped inside diamond lattice, the potential energy and trapping site were determined on the basis of semi-empirical PM3 MO method combined with the direct dynamics calculation. The calculation shows that these atoms are trapped deeply in the tetrahedral site. Reduction of the spin density from 0.977 at 0 K to 0.975 at 300 K suggests that the donors are enhanced due to increase in both amplitude and frequency of thermal vibrations of the trapped atoms. (C) 2000 Elsevier Science B.V. All rights reserved.