Fe-I has three 4-fold rotation axes, given an EFG = 0 from theorem 2. If we don’t make a distinction between Fe-IIa and Fe-IIb, they would also have three 4-fold rotation axes. If they are distinguished, both have one 4-fold rotation axes and two 2-fold rotation axes. From theorem 1, any of these 2-fold and 4-fold rotation axes can be chosen as z-axis PAS, but only the 4-fold rotation axes could be used to have a symmetry parameter =0.