The calculations presented here have successfully explained some of
the remaining difficulties with the structural models of the carbon
and di-carbon defects. Our calculated LVMs for the C 
defect are in
reasonable agreement with experiment, and the near equality of the
three C-Si lengths results in the two almost degenerate local
vibrational modes. The calculated infra-red intensity ratio of the
two LVMs gave a ratio of 1.5-1.7:1 in fair agreement with the
experimental result of 2:1. The calculated energies for the
migration/reorientation barriers lead to an upper bound of 1.10 eV, in
good agreement with experiment.
The B-form of the neutral C -C 
defect gave rise to six
local modes, only two of which have been detected experimentally by
PL. Two of these have very high frequency, and probably could not be
detected because of large anharmonicities which would lead to a very
broadened absorption. Two of these modes having 
symmetries lay
around 560 cm 
. These involved almost independent movements of
the C atoms with there neighboring Si atoms, excluding the
interstitial. The isotope shifts of the vibrational modes are in
excellent agreement with experiment. Investigations into the relative
energies of the configurations of the di-carbon defect with charge
state confirms that the stability of the A-form over the B-form
increases with charging.