We investigate the stability and local vibrational properties of H 2 and H2∗ complexes in Si through first-principles pseudopotential calculations within the local-density-functional approximation (LDA) and the generalized gradient approximation (GGA). We find the energy difference between H 2 and H2∗ to be less than 0.05 eV per H atom, testing supercells containing up to 216 atoms. For the [100],[110], and [111] orientations of the H 2 molecule at or near a tetrahedral site, the vibrational frequencies by the GGA for the stretch mode lie in the range of 3556–3643cm−1, close to the experimental value of 3618cm−1, while they are underestimated by about 300cm−1 in the LDA. A new local mode at 650–700cm−1 is predicted for the H 2 molecule.