You may get the Cd-Se bond length from bulk CdSe structure that is available in the VNL database, go to Add->From Database-> Search for CdSe-> Add the structure to the Stash in the Builder -> Measure the bond length. Using the initial guess for the bond, you can further do geometry optimization for CdSe tube in the VNL-ATK to get the actual equilibrium separation distance between Cd and Se atoms.