The properties of vacancy aggregates in crystalline material were studied by using density-functional molecular dynamics simulations involving large periodic super-cells, as well as approximate  ab initio Hartree-Fock calculations for molecular clusters. The stability and properties of aggregates containing up to 7 vacancies were considered. The ring hexavacancy, V6, in particular, exhibited remarkable properties. It was theoretically predicted to be very stable, trigonal, planar, electrically inactive, and essentially invisible to photoluminescence and infra-red absorption spectroscopy. However, it could be Raman-active. It was concluded that this defect was probably a gettering center, and was the nucleus or precursor of a range of extended defects. The V6 defect was expected to resemble small prismatic dislocations such as those nucleated by vacancy condensation. It was also the smallest possible dislocation loop. Although it was unclear how an isolated V6 could become an extended structure, the molecular dynamics calculations showed that V6 should trap more vacancies.

J.L.Hastings, S.K.Estreicher, P.A.Fedders: Physical Review B, 1997, 56[16], 10215-20