The irradiation of c-Si produced self-interstitials (Sii), which interacted with substitutional C to produce interstitial carbon (Ci). The latter was mobile at room temperature and interacted with interstitial oxygen (Oi) to form the C3 center (CiOi). If enough Sii's were provided, C3 trapped one more Sii and formed the C4 center (SiiCiOi). The first 2 defects, i.e., Ci and CiOi, were abundantly studied by both experimentalists and theorists. Their electrical, optical and magnetic properties were very well understood. On the other hand, the C4 defect was incompletely characterized experimentally and lacked a precise theoretical description. The results of first-principles calculations of the configurations, binding energies, vibrational spectra and estimated gap levels of these defects - with emphasis on the C4 center - were presented here. There were 3 configurations of C4, labelled (a), (b), and (c). All of them exhibited vibrational modes which were consistent with the existing data. However, the origin of the new infra-red line (~760/cm) associated with C4 was different: it was O-related in C4(a) and C4(c) but C-related in C4(b). The 3 configurations had distinct gap levels. C4 Defect and its Precursors in Si - First-Principles Theory. D.J.Backlund, S.K.Estreicher: Physical Review B, 2008, 77[20], 205205 (8pp)