The coordinates for Pb in http://www.cryst.ehu.es are : (1/2,0,z) (0,1/2,-z) . Our instructor in the video said that the undefined coordinates can be set between [0,1] . So does that mean that I can choose whatever number I want between that interval. Can I for example put for the Pb coordinates (1/2,0,1/10) (0,1/2,2/10) and the material will still be Litharge? I don`t find that logic. Can anyone explain to me how to choose those undefined coordinates please.