[Wien] Problems of calculating the effective U and setting shift during the supercell procedure

[晨晨]

Dear WIEN2k experts and users,
 
I am a beginner of WIEN2k. The version is WIEN2k_21.1. Now, I am learning to calculate the effective U following the note “Calculating the effective U in APW methods. NiO” by Georg K.H. Madsen and Pavel Novak. I have two problems about this topic. 
 
One is about generating a supercell. I generated the NiO.struct as the following: first get the NiO.cif from the external database, then by cif2struct to generate NiO.struct. During the supercell procedure, I set the numbers of xyz direction are all 2, but I am not sure how to set the optional shift (I think in this case I should set the optional shift are all 0?).
 
The other problem is if a supercell is required when we calculate the effective U. I am considering the material of Fe3Al, in which exist 12 Fe atoms. In this case, do I have to generate a supercell of Fe3Al to calculate the effective U of Fe? Or could I set one of Fe atoms as an impurity. 
 Thank you for your reply in advance. 
Best regards,
 Yu
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