English: Computed structure of a hypothetical (H
2O)
100 "icosahedral water cluster".

An enclosed central (H
2O)
20 dodecahedral water cluster is shaped like a regular dodecahedron (not shown), which has 20 vertices.

That plus a surrounding layer of 80 H
2O forms this (H
2O)
100 icosahedral water cluster (left). The outer polyhedral shape (right) is a rhombicosidodecahedron, which has 60 vertices (so this cluster must include 20 more H
2O somewhere).

The next layer would add 180 more H
2O, forming icosahedral (H
2O)
280.