[Wien] Impurity sites in supercell

[Shu Miao]

Dear Wien users
I have a question in building a supercell. As said in the UG, one should
make some change on an atom in the expended cell, e.g. 2x2x2, to build a
supercell compatible with symmetry operations. Assuming 4 atoms in the
original cell, then we have 32 atoms in the expanded cell. If I replace
atom OA with an impurity IA, the final structure will be 31 original
atoms plus IA. However, IA has equivalent positions at (x/2, y/2,
z/2), where (x,y,z) are the equivalent positions of OA in the original
cell. These sites of IA are within a 1/8 corner of the expanded cell. So
what we create actually is a localized cluster of IAs. Is this what the
supercell program surposed to do, or I do something wrong? Is there a way
to create even distribution of the impurity in the expanded cell or just
one impurity site in the super cell? Many people talking about the
applicaiton of the supercell in the mailing list, but I didn't find
concerns about the distribution of impurities within the supercell.
Why? Because this won't make difference?
Please advise.
Many thanks

Shu Miao