Fe-I: We try to imagine the unit cell copy and pasted around the Fe-I atom, and we determine that there are three 4-fold symmetry axis along the edges of the unit cell. There are also three 2-fold symmetry axes along the diagonals passing through the Fe-I atom in each of the sides of the unit cell. Finally, there are four 3-fold rotation axes that coincide with the line connecting the Fe-I atom with the N atom. We can determine that the EFG tensor is zero since there are two or more 3-fold rotation axes.

Fe-II: We can identify one 4-fold symmetry axis coinciding with the line connecting the Fe-II atom with the N atom. There are also two 2-fold axes that lie diagonal (they connect Fe-II with Fe-I). Lastly, there are two more 2-fold axes that pass ‘horizontally’ and ‘vertically’ through the Fe-II atom. We determine that the z-axis of the PAS will be the 4-fold axis, and eta = 0.