The structure of a single vacancy was studied by performing extensive first-principles calculations. It was unexpectedly found that there existed 2 distinct distortions which were associated with the vacancy. These alternative forms required essentially identical formation energies under zero pressure. The two distortions could be distinguished by their differing relaxations, formation volumes and formation enthalpies.

Vacancy in silicon revisited A.Antonelli, E.Kaxiras, D.J.Chadi: Physical Review Letters, 1998, 81[10], 2088-91