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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?

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Did you check that the bulk structures are stable? If they are, then the Coulomb part should be working, and the problem might be in your model.

This type of problem is actually relatively common. If you use potentials that have been fit for bulk materials i.e. perfect crystals, you're only studying a small range of the possible configuration space: the atoms are perfectly bound, and have the perfect bond angles. Going to nanostructures, however, the atoms on the surface have fewer bonds than the ones inside, and all bets are off as to how the force field will behave.

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).

A more sophisticated potential, like Tersoff, might yield more reliable results. I even found a Fe-O potential published last year: J. Phys.: Condens. Matter 31 215401 (2019)

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  • $\begingroup$ Good point, it's an interesting question. A similar problem happens when studying crystal clusters, if you model the dynamics without periodic boundary conditions the cluster will fall apart. $\endgroup$
    – Cody Aldaz
    Jul 28, 2020 at 18:36
  • $\begingroup$ Related, just watched a talk discussing how different molecular mechanics parameters are required for folded and unfolded protein structures. $\endgroup$
    – Cody Aldaz
    Jul 28, 2020 at 21:15
  • $\begingroup$ Hey @CodyAldaz, Can you share the link if it is available online? $\endgroup$
    – Magic_Number
    Jul 30, 2020 at 11:29

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