Fe-I: There are 3 4-fold rotation axis (x,y and z).
-Theorema 1 gives that every axis can be chosen as a z-axis and eta=0.
-Theorema 2 gives that the EFG tensor is zero
Fe-II (a and b): There are 2 2-fold rotation axis (x and y) and 1 4-fold rotation axis (z)
-Theorema 1 gives that a 2-fold rotation axis can be chosen as a z-axis