  We show that nanoparticles can have very rich ground state chemical order.
This is illustrated by determining the chemical ordering of Ag-Au 309-atom
Mackay icosahedral nanoparticles. The energy of the nanoparticles is described
using a cluster expansion model, and a Mixed Integer Programming (MIP) approach
is used to find the exact ground state configurations for all stoichiometries.
The chemical ordering varies widely between the different stoichiometries, and
display a rich zoo of structures with non-trivial ordering.
