For the Fe-I atoms, there are three 4-fold rotational axis. Thus, by theorem 1, any of these three rotational axis are the z-axis of a PAS with η=0. By theorem 2, since there are more than two 4-fold rotational axis, the EFG tensor must be zero.

For the Fe-II atoms, we have two 2-fold rotational axis and one 4-fold rotational axis. Thus, by theorem 1, the 4-fold rotational axis is the z-axis of a PAS with η=0 and the other two 2-fold rotational axis can be chosen as the z-axis of a PAS with η≠0. We cannot apply theorem 2 here, so the EFG tensor will not necessarily be zero.