Abstract: The electronic structures of impurity in silicon have been studied by SCF-X_α-SW method, it is shown that the electronic structure is depending strongly on the atomic radius in the cluster. We have proposed two rules to determine the atomic radius:1) Adjusting the atomic radius of master element according to the band properties of crystal;2) Determining the radius of impurities in terms of minimizing the total energy of cluster.The calculations of electronic structure of Si:Pd and Si:Pd systemsshave demonstrated that these rules are efficient.