The third order approach in which the perturbing influence of crystal field potential and electron correlation effects is taken into account is illustrated numerically. The contributions to the dipole and quadrupole mechanisms are analyzed in the case of the Gd 3 + ion, and the relative importance of various second and third order contributions is established. Special attention is directed to the properties of the perturbed functions that are generated for Pr 3 + , Eu 3 + , Gd 3 + and Tm 3 + . In particular, the behavior of a new function that is characteristic for the model of energy transfer, and that describes all single excitations to the n'f one electron states via the even part of crystal field potential, is demonstrated.