Are MoMo molecules hard spheres?

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This Metropolis Monte Carlo simulation treats dimolybdenum tetraacetate molecules, referred to as MoMo, adsorbed on the (10.4) surface of the bulk insulator calcite. MoMo molecules are modelled as hard spheres with a diameter of $0.9\,\mathrm{nm}$ . The molecules are confined to a grid with a spacing given by the Ca-Ca distance on calcite (10.4) (black grid in the figure). For $[0\bar 10]$ pointing in $x$ direction the $[42\bar 1]$ points in $y$ direction. The calcite slab consists of $1600$ unit cells $(648\,\mathrm{nm}^{2})$ . The maximum number of MoMo molecules this slab can accomodate is $800$ , i.e., the maximum molecule density is $1.23\,\mathrm{nm}^{-2}$ .

The center positions of the spheres (blue dots in the figure) are shown every $n$ simulation steps. Here, a single simulation step is the attempt to move a random sphere to a random adjacent position. Note that an ordered arrangement is used as starting configuration. Hence, equilibration may take many simulation steps when $N$ is large.

Pause simulation Disable grid

The sphere-to-sphere distances $d$ are evaluated every $n$ steps and the total distance count is added to the histogramm below. The grid can be disabled for a generic hard-spheres simulation in 2D. In that case, the distance count in the histogramm is weighted with sphere diameter over sphere-to-sphere distance.