The previously suggested procedure for the account of boundary conditions for clusters of covalent crystals has been realized on the basis of high-order finite-difference method. In cluster approximation the peculiarities of the electron structure and space structure of regular crystals (GaAs, Si) as well as the impurity centers (GaAs:Si, Si:P, Si:S) are studied. We consider the defect stability, its probable reconstruction and also the influence of the center charge on its stability. The reasonably good agreement with the results of experiment as well as with the conclusions of crystalline approach (expanded cell method) has been obtained.