Irradiation of Si with highly energetic particles creates vacancies, interstitials, multivacancies, and other complex defects. Several multivacancies have been identified from EPR signals by assuming that their localized states can be constructed as linear combinations of the so-called dangling bonds on the surrounding atoms. Theoretical investigations have so far been carried out only for divacancies, most of them using Extended-Hückel Theory (EHT) cluster calculations. model have been indirect and tenuous. In this paper, we report a theoretical scheme which gives a systematic description of the electronic energy-level structure of multivacancies of any size and shape. We obtain a direct connection between the results of Green's-function calculations and the dangling-bond model and reformulate the latter in a way that gives it quantitative predictive power. We identify the driving force for vacancy clustering and suggest that some of the observed rod-like defects may be vacancy chains. In the case of self-interstitials, we report self-consistent Green's-function calculations whose results are in sharp contrast with earlier results obtained by extended-Hückel cluster calculations. These results and other theoretical considerations led us to suggest that self-interstitial migration in Si is more likely to occur through the low-electron-density channels rather than through bond sites, as suggested earlier on the basis of the extended-Hückel calculations. © 1983.