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3:14 PM
You should just look at the local density: for a finite volume centered around R (e.g. a small box), compute the number of particles in the box and normalize by the volume. The solid nanoparticle will show up in such a plot as a region of high density, whereas the gas shows up as low-density area. The box size is a parameter, so you should try out several values to figure out a good compromise.

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