The successful application of quantum mechanics to atomic spectroscopy
in the 1920’s led inevitably to studies directed to the understanding
of similar phenomena in molecules and crystal lattices. It was Bethe (1)
who, in 1929, wrote a now classic paper in which the framework of crystal
field theory was set out. In this approach, the energy of an ion is taken to
be modified by a potential field set up by its neighbors that are assumed to
be described by point charges, or simple dipoles. For idealized systems
involving high symmetry, Bethe made full use of group theory to establish
the nature of crystal field terms arising from a given configuration. His
work was put to good use by Van Vleck (2) in the field of magnetism. Van
Vleck and also Penney and Schlapp (3) developed the theory of the magnetic
susceptibilities of transition metal ions including some particularly
fruitful studies of the lanthanides.