Using a Tersoff‐type empirical potential energy function, the free energy of formation for microvoids in silicon containing from 1 to 57 vacancies was calculated as a function of temperature and vacancy supersaturation. The results apply equally well to microvoid nucleation during crystal growth or, at low temperatures, as a consequence of ion implantation. The results indicate that homogeneous nucleation is an unlikely process for the crystal growth case where heterogeneous nucleation via adsorbate attachment to the microvoid surface is a very likely process. ©1995 American Institute of Physics.