The vibrational modes of H2 molecules were predicted by using a first-principles method, and were compared with experimental data. The isolated molecule was found to lie at a Td interstitial site, oriented along [011], and was infrared-active. The rotational barrier was equal to at least 0.17eV. The molecular frequency was a sensitive function of the cage size, and increased almost to equal the gas value for cages which were about 50% larger than the Td site. It was suggested that observed Raman-active modes around 4158/cm were due to molecules within voids.

B.Hourahine, R.Jones, S.Oberg, R.C.Newman, P.R.Briddon, E.Roduner: Physical Review B, 1998, 57[20], R12666-9