[Wien] Tetrahedral Symmetry

[fabella]

Hello users,

I have a question regarding tetrahedral symmetry.  For the vacancy 
formalism of Group IV elements it is well known that there is 
tetrahedral distortion during relaxation.  This (Tetrahedral symm) Td 
is reduced to D2d, were there are normally 2 shorter bonds and 4 longer 
bonds between atoms forming the tetrahedron.

-The Question-
I have relaxed a Group IV vacancy and the variation between short and 
longer bonds is about 10 pm (?), so not very much distortion?  In 
addition, the relaxation has decreased the volume of the vacancy by 
50%.  So relaxation, the forces are sufficiently minimized, but the 
tetrahedral distortion is small!

May I be enlightened by any explanations why this is so?

Thank You for answering this question in advance!!!

Wesley