Many-electron treatment of the off-center substitutional O in Si

We present for the first time a quantitative justification for the occurrence of a pseudo-Jahn-Teller effect at the neutral, isolated substitutional oxygen in Si, the origin of the well-known Si A center. We show that, in this case, multiplet effects must necessarily be taken into account. Our results also give values for the hyperfine interactions in the excited triplet state which are in excellent agreement with experiment.