You may want to study Mg-doped GaN as this would be a p-type semiconductor but you certainly would not want to study antiferromagnetic coupling between Mg2+ ions as Prof. Blaha has pointed out. However if you were interested in Mg-doped GaN you would probably want a low level of dopant and hence a large supercell. Or you may have been asked to look at manganese (Mn) rather than magnesium (Mg). You would then need to decide on the oxidation state of the Mn ion, possibly Mn3+ on a Ga site.
Elaine A. Moore Department of Chemistry and Analytical Science The Open University UK -- The Open University is incorporated by Royal Charter (RC 000391), an exempt charity in England & Wales and a charity registered in Scotland (SC 038302).
