Hybrid density-functional study of the vibrational frequency of a H2 molecule at the tetrahedral site of silicon

Ab initio molecular orbital calculations based on cluster models were performed to investigate the vibrational frequency of a H-2 molecule at a tetrahedral (T) site of Si, whose value has been controversial among theoretical and experimental studies. To check the validity of the calculations, we examined the effect of the electron correlation basis set, cluster size, and anharmonicity. Among a variety of techniques to include the electron correlation, a hybrid method that combines the Hartree-Fock (HF) method and the density-functional approach was found to give reliable results, while the HF method employed in previous studies overestimates the frequency. The present study indicates that the vibrational frequency of a H-2 at the T site is substantially lower than that of the free molecule, and then provides important information on the identification of observed frequencies of H-2 in Si. [S0163-1829(98)06436-4].