The bonding and dynamics of Mg in pyrope are investigated using density fun
ctional theory calculations. The potential which the Mg experiences is high
ly anisotropic and in one direction is strongly anharmonic. Frequencies cor
responding to displacements of a Mg atom along various directions have been
obtained from frozen phonon calculations. From the shape of the computed p
otential, it follows that there is no subsite dodecahedral ordering of the
Mg around the 24c site in pyrope.