1. Fe-I :- x,y,z-axes have 4-fold rotation symmetry. EFG is zero in this case.
2. Fe-II :- z-axis has a 4-fold rotation symmetry; this can be the z-axis for the PAS and eta is zero.
x and y- axes have 2-fold rotation symmetry; these two axes can also be chosen as the z axis of the PAS.