  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.
