The FeI atom has three 4-fold rotation axes: the x, y and z axes. From theorem 1 it follows that all these axes can be chosen as the z-axis of the PAS so that eta = 0. In addition, from theorem 2 it follows that the EFG tensor is zero.

The FeII atom has two 2-fold rotation axes : x and y, and the z-axis is a 4-fold rotation axis. The z-axis can then be chosen as the z-axis of the PAS with eta = 0 as follows from theorem 1.