In an ``ideal'' unrelaxed and neutral vacancy V0, four dangling sp3 hybrids point inwards. The combination of these orbitals in a tetrahedral symmetry gives rise to a nondegenerate state belonging to the a1 representation of the Td symmetry group, and to another triply degenerate state belonging to the t2 representation . The a1 state has the lowest energy and is filled by two of the four electrons of the dangling bonds. The other two electrons occupy the localized t2 state. The energy levels induced by these states fall well inside the band gap. The a1 state ofen appears as a resonance state near the top of the valence band.
Such a partially filled degenerate electronic state t2 is unstable under Jahn-Teller distortion which lowers the symmetry of the structure. The t2 triply degenerate state then splits into a nondegenerate a1 state and a doubly degenerate e state. The two electrons that occupied the t2 state will therefore be in the a1state, and the e state will remain unoccupied, as illustred in figure (5.1).
It has to be mentioned that the many-electron effects are negligible with respect to the jahn-Teller effect itself . Therefore, the one-elecron picture of the tight binding model may be suitable to reproduce the energy levels associated with the vacancy defect. Song et. al.  studied intrinsic point defects in crystalline silicon by tight binding molecular dynamics. In particular, the lattice distortion induced by Jahn-Teller effect is well reproduced, and corresponding energy levels are obtained in the band gap.
In experiment, a so called GR1 band is measured, associated with an optical transition at the neutral vacancy V0 between the levels mentioned above. This signal is observed in an irradiated or implanted sample, which suggests that vacancies are created during this process and remain relativly stable, untill annealing at high enough temperature is carried out.