Two kinds of hydrogen dimers have been observed in c-Si: the interstitial H2 molecule and the H*2 complex. Theory predicts that H2 and H*2 are within a few tenths of an eV of each other, but the two centers have never been observed to coexist. Instead, the irradiation of samples rich in H2 induces its conversion into H*2. No such conversion has been thermally induced, and H*2 is seen only in irradiated material. In the present paper, ab initio molecular-dynamics simulations demonstrate in real time how these reactions occur. A vacancy (V) or a self-interstitial (I) break up interstitial H2resulting in the (V, H, H) or (I, H, H) complex. Then V-I recombination forms H*2as in (I, H, H)+V → H*2.