Using density-functional-theory calculations, we have identified new stable configurations for tri-, tetra-, and penta-vacancies in silicon. These new configurations consist of combinations of a ring-hexavacancy with three, two, or one interstitial atoms, respectively, such that all atoms remain fourfold. As a result, their formation energies are lower by 0.6, 1.0, and 0.6 eV, respectively, than the ``part of a hexagonal ring'' configurations, believed up to now to be the lowest-energy states.
@article{arxiv.cond-mat/0405152,
title = {Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations},
author = {D. V. Makhov and Laurent J. Lewis},
journal= {arXiv preprint arXiv:cond-mat/0405152},
year = {2009}
}