Ab initio total-energy calculations based on the local-density-functio
nal, pseudopotential, and super-cell approximations are performed to i
nvestigate carbon defects in silicon. The geometry and the formation e
nergy of substitutional and impurity-vacancy defects are studied inclu
ding the relaxation of nearest and next-nearest neighbors. Results for
substitutional carbon appear to be consistent with a recently suggest
ed reinterpretation of the available experimental formation energy dat
a. Results for the interaction energy between a carbon atom and a sili
con vacancy predict a small binding energy of 0.19 eV.