Calculations of space geometry and electron structure of self-interstitial atoms (IAs) in Si have been made in cluster approximation by using self-consistent discrete variational Xα-method (DV-Xα), semiempirical MINDO/3 method and molecular dynamic method (MD). The results of DV-Xαcalculation show that a high state density in the band gap and various spin states are observed for interstitial complexes. As follows from the analysis of the data obtained, the methods resulting from neglecting the differential overlapping, can not be used for satisfactory description of IAs properties. MD calculations prove the existence of a new stable configuration, oriented in one of the plane equivalent to (110) and containing a pair of IAs in tetrahedral cavity. The results of theoretical analysis are compared with those available in literature on the properties of IAs in Si.