text: When interatom bonding does not have strong directional preferences, it is not unusual for atoms to gravitate toward a kissing number of 12 nearest neighbors. The three most symmetric ways to do this are by icosahedral clustering, by crystalline face-centered-cubic (cuboctahedral) and hexagonal (tri-orthobicupolar) close packing.
  Icosahedral arrangements, typically because of their smaller surface energy, may be preferred for small clusters. However, the Achilles' heel for icosahedral clustering  is that it cannot fill space over large distances in a way that is translationally ordered, so there is some distortion of the atomic positions, that is elastic strain. De Wit pointed out that these can be thought of in terms of disclinations, an approach later extended to 3D by Yoffe. The shape is also not always that of a simple icosahedron, and there are now several software codes that make it easy to calculate the shape.
  At larger sizes the energy to distort becomes larger than the gain in surface energy, and bulk materials (i.e. sufficiently large clusters) generally revert to one of the crystalline close-packing configurations. In principle they will convert to a simple single crystal with a Wulff construction shape. The size when they become less energetically stable is typically in the range of 10-30 nanometers in diameter, but it does not always happen that the shape changes and the particles can grow to millimeter sizes.