The static and dynamic properties of interstitial H2, HD and D2 molecules in crystalline Si were obtained from ab initio molecular-dynamics simulations with atomic-like basis sets. The static (T = 0) calculations agree with those of most other authors: the centre of mass of H2 was at the tetrahedral interstitial site, the molecule was a nearly-free rotator, and the activation energy for diffusion was 0.90eV. However, these results fail to explain a number of experimental observations, such as why H2 was infra-red active, why the expected ortho/para splitting was not present, why the symmetry was C1, why the piezo-spectroscopic tensors of H2 and D2 were identical or why the exposure to an H/D mix results in a single HD line which was not only at the wrong place but also much weaker than expected. The static calculations were here extended so as to include the constant-temperature dynamics for H2 in Si. At temperatures greater than 0K, the centre of mass of the molecule no longer remains at the tetrahedral interstitial site. Instead, H2 ‘bounced’ off the walls of its tetrahedral cage and exchanges energy with the host crystal. The average position of the centre of mass was away from the tetrahedral interstitial site along <100>. Under uniaxial stress, the centre of mass shifts off that axis and the molecule has C1 symmetry. The H-H stretch frequency calculated from the Fourier transform of the v-v autocorrelation function was close to the measured one. Since the potential energy experienced by H2 in Si near the tetrahedral interstitial site was very flat, it was argued that H2 should be a nearly free quantum mechanical rotator. Up to room temperature, only the j = 0 and j = 1 rotational states were occupied, H2 resembles a sphere rather than a

dumb-bell, the symmetry was determined by the position of the centre of mass and HD was equivalent to DH in any symmetry. The rapid motion of the centre of mass implies that an ortho-to-para transition will occur if a large magnetic moment was nearby. Several candidates were proposed. Since nuclear quantum effects were not included in the present calculations, it was not possible to decide whether the observed vibrational spectrum of H2 resulted from a tunnelling excitation.

Dynamics of Interstitial Hydrogen Molecules in Crystalline Silicon. S.K.Estreicher, K.Wells, P.A.Fedders, P.Ordejón: Journal of Physics - Condensed Matter, 2001, 13[29], 6271-83