Molecular hydrogen in silicon: A path-integral simulation

Molecular hydrogen in silicon has been studied by path-integral molecular dynamics simulations in the canonical ensemble. Finite-temperature properties of these point defects were analyzed in the range from 300 to 900 K. Interatomic interactions were modeled by a tight-binding potential fitted to density-functional calculations. The most stable position for these impurities is found at the interstitial T site, with the hydrogen molecule rotating freely in the Si cage. Vibrational frequencies have been obtained from a linear-response approach, based on correlations of atom displacements at finite temperatures. The results show a large anharmonic effect in the stretching vibration, omega_s, which is softened with respect to a harmonic approximation by about 300 cm^{-1}. The coupling between rotation and vibration causes an important decrease in omega_s for rising temperature.