  Rattling motion of fillers in cage materials has been of great interest for
their import roles in superconductivity and thermoelectric applications. The
standing waves of the rattling oscillations are normally lower in energy than
the propagating waves of the acoustic phonons, thus exert large influences on
the configuration of phonon dispersions as well as the associated thermal and
electrical properties. Although it has been extensively studied, the origin of
the low energy soft modes is still not clear. In the present paper, we show
that van der Waals-type interactions are predominant between fillers and their
surrounding cage frameworks, which explains the origin of the low energy modes
in cage materials as a universal rule. Mass, free space and chemical
environment of guest atoms are shown to be the most important factors to
determine the three dimensional van der Waals-type interactions. The present
work is mainly focused on type-I clathrates, skutterudites and pyrochlores.
