We have performed systematical ab initio studies of the structural properti
es of SixGe1-x alloy. To simulate the disordered alloy we use supercells wh
ere the Si and Ge atoms are randomly placed with the constraint that the pa
ir correlation functions agree with their values for a perfect random alloy
within a given tolerance. We obtain that the Si-Si, Si-Ge, and Ge-Ge bond
lengths dependence with composition varies only slightly for the different
kinds of bonds, with topological rigidity parameters between 0.6 and 0.7.