# Why do interaction potential parametrizations of bulk systems not work for corresponding nano-systems?

I am studying several thermodynamics properties of crystalline iron oxides ($$\ce{Fe3O4}$$, $$\gamma-\ce{Fe2O3}$$) in their solid phases by MD simulations. At the moment, I am facing many difficulties in simulating the nano particles.

The crystal structures I am using are coming from first principle simulations and interaction potential parametrisations (Buckingham potential for short range and Coulombic potentials for Long range) developed for bulk iron oxides. Clearly the modelling is properly working for bulk iron oxides. However, when applied to nano-particles, the nano-particles become amorphous at any temperature.

In my opinion, the lack of Coulombic contribution in the nano-system (for bulk, the contribution is large, as the simulation box is periodic in each dimension) is causing this behavior.

Has anyone ever faced such issue while studying crystalline nano-structures?

You're using the Coulomb-Buckingham potential, which is a pure pair potential $$V(r) = A \exp(-Br) - C r^{-6} + q_1 q_2/ 4 \pi \epsilon_0 r$$. I'm not an expert on iron nanoparticles, but I would assume that this kind of a pair potential is not expected to be very accurate as it neglects the effects of distance (the bonds might become stronger at shorter internuclear distances) and geometry (bond angle affects the strength of the bond).