[Wien] Simulating charged defect

[suman mandal]

 Dear Prof. Blaha and WIEN2k users,

I am new with this code, having a basic idea. I have started a problem
for that I need to calculate the defect formation energy for oxide
materials (like ZnO, MgO etc. ). I understand that by removing one
atom from the structure file, one can simulate the vacancy with
supercell. But i really do not understand how to simulate a vacancy in
a charged state ( say -2 fo Zn vac) in supercell method. please help
me.
thanks in advance,

Suman Mandal
Saha Institute of Nuclear Physics.
Kolkata, India.