Surface and size effects for impurities in Si clusters

Silicon clusters saturated with hydrogen atoms are used to calculate charge distributions and various properties of interstitial hydrogen as a function of the host-saturator bond length. The clusters range in size from Si₂{H}₆$ to ${Si}₄₄H₄₂. Most of the calculations are done at the approximate ab initio level, using the method of partial retention of diatomic differential overlap. The results are complemented with ab initio Hartree-Fock calculations. It is shown that the surface dependence is minimal when the host-saturator bond length is set to minimize the total energy of the clusters. This result is consistent with the one obtained in the case of diamond clusters. A set of ``rules'' for calculating properties of impurities in semiconductor clusters is proposed.