Calculation of the crystal field parameters (CFP) of the rare-earth (RE)
ions in solids is a formidable problem and until recently no robust
ab-initio method to calculate them was available. The main problem is
that in most approaches the 4f electrons of RE are allowed to interact
with the nonspherical electron density they create themselves and this
false selfinteraction completely distorts the crystal field. The second
problem is that no ab-initio method is capable to determine correctly
hybridization of the 4f levels with ligand electron states.
