...hard-spheres

In fact, the difference in free energy of the face-centered cubic and hexagonal close packed ( hcp) structures is so small that it is not known which one is the more stable. Our calculations were performed for the fcc structure, but the results would have been virtually the same for the hcp structure. Other crystal structures (e.g. simple-cubic and body-centered cubic) can be safely ignored because they have a much lower maximum packing density.

Peter Bolhuis
Tue Sep 24 20:44:02 MDT 1996