We used GTH pseudopotentials to replace the core electron density. For the valence electrons, we used the GTH basis set of triple zeta (gth-tzvp) quality for the N and six C atoms -three attached to the N, and the other three making dangling bonds with the vacancy, and of double zeta (gth-dzv) quality for the remaining atoms. This combination of basis sets (gth-dzv + gth-tzvp) was employed for all the calculations described in this work, if not mentioned otherwise.