It was shown that a 1-electron linear Jahn-Teller coupling model for the negative charge state of the Si vacancy could not predict the observed Jahn-Teller distortion. It was proposed that quadratic Jahn-Teller coupling should be included. Two typical schemes were described and it was shown that they could account for experimental observations. The role of the Coulomb interaction among the electrons was investigated. It was shown that, even though this interaction could be strong enough to affect the energy significantly, it had only a slight effect upon the wave functions.

F.G.Anderson, F.S.Ham, G.Grossmann: Physical Review B, 1996, 53[11], 7205-16