The pseudo-Jahn-Teller effect for isolated neutral substitutional oxygen and nitrogen impurities in silicon is examined. The author demonstrates the effect of having the electron-lattice coupling both between the antibonding a1 state and the antibonding t2 manifold and within the antibonding t2 manifold use the same tau 2 modes of distortion. He then makes a qualitative analysis of the conditions under which a (001) distortion is a lower-energy configuration than a (111) distortion. In addition, he shows how a quadratic Jahn-Teller coupling within the antibonding t2 manifold with use of tau 2 distortion modes can lead to a minimum-energy configuration involving a (001) distortion.
 

Physical Review B, 39 5392-6, 1989.