Results of a molecular-dynamics computer simulation are presented for atomic relaxations and relaxation energies for self-interstitials in a silicon crystal. The Stillinger-Weber model potential containing two- and three-body terms is used and is expected to be more realistic than a simple Keating potential. The host crystal is represented by a cluster of 800 atoms, and the additional silicon atom was embedded in various interstitial sites near the center. The whole assembly was then periodically continued to fill the entire space. It is found that significant atomic relaxations occur in a shell of a radius 11 a.u. and decay exponentially. In fact the relaxation is oscillatory in nature and also nonuniform within some shells. The calculated formation energies of vacancy and self-interstitials at equilibrium show trends which are in agreement with the self-consistent field total-energy calculations. These energy values are also in agreement with the known self-diffusion activation energy. From calculated formation energy values, we are able to draw the conclusion that the tetrahedral-site interstitial can be most readily formed. The hexagonal-site interstitial, on the other hand, is most repulsive. The migration from tetrahedral to dumbbell interstitial site appears to be most favorable. © 1987 The American Physical Society.