  The entropy per particle in most Monte-Carlo simulations is size dependent
due to correlated energy fluctuations. Guided by nanothermodynamics, we find a
constraint for the Ising model that enhances the fluctuations and lowers the
free energy, while making the entropy homogeneous, additive, and extensive.
Although the average interaction energy becomes size dependent, the resulting
distribution of energies provides a mechanism for the heterogeneity found in
the dynamics of many materials.
