Lattice distortions and electronic structure in the negative silicon vacancy

We show that a one-electron linear Jahn-Teller coupling model for the negative charge state of the silicon vacancy cannot predict the observed Jahn-Teller distortion. We propose that quadratic Jahn-Teller coupling should be included. We detail two representative schemes and show how they may account for experimental observations. We investigate the role of the Coulomb interaction among the electrons and show that even though this interaction can be strong enough to affect the energy significantly, it has only a slight effect on the wave functions.