  The isolated substitutional gold impurity in bulk silicon is studied in
detail using electronic structure calculations based on density-functional
theory. The defect system is found to be a non-spin-polarized negative-U
centre, thus providing a simple solution to the long-standing debate over the
electron paramagnetic resonance signal for gold in silicon. There is an
excellent agreement (within 0.03 eV) between the well-established experimental
donor and acceptor levels and the predicted stable charge state transition
levels, allowing for the unambiguous assignment of the two experimental levels
to the (1+/1-) and (1-/3-) transitions, respectively, in contrast to previously
held assumptions about the system.
