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Currently I am working a molecular simulation to determine phases of an argon NPT ensemble using Lennard Jones potential. Mainly I use the radial distribution function to determine solid, liquid, or vapor state by investigating the structure.

A problem comes in when I notice that there are some states which should be in vapor form that exhibit the radial distribution function of liquid (showing a widen peak). I surmise that there may be due to the high density gas especially above the critical point.

Is there any way to decisively differentiate the liquid and vapor phase?

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What's your N? By the way "some states which should be in vapor form" --- why do you think they should be in a vapor form? – Yrogirg Dec 25 '12 at 4:13

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Not a complete answer, but hope it will help. Have you heard about SklogWiki? Among others it has a nice article about Lennard-Jones model.

The thing is that you are not dealing with argon, instead you are dealing with a model Lennard-Jones fluid. So it is wrong to suggest that phase transitions of the two perfectly coincide. Molecular potential assumed to be the true potential of argon is referred to as Aziz potential ("A highly accurate interatomic potential for argon", paywalled).

I would suggest you to compare your results with ones already known for LJ fluid, for example SklogWiki links to a work by JP Hansen, L Verlet "Phase Transitions of the Lennard-Jones System."

Apart from this I have a couple of remarks. You mention "states above critical point" --- there is no gas or liquid above the critical point, it is a state known as "supercritical fluid". It is normal you can't distinguish vapor from liquid there. The other thing I would be concerned of is the coexistence of vapor and liquid in the simulation. If it takes place everything is wrong. However I have never encountered this and I don't know how to diagnose such a nasty case.

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Thank you for the great info. I seem to be ignorant of the existence of the supercritical fluid. Maybe the graph of density versus pressure at fixed temperature is more useful to observe the transition – Sheng Dec 25 '12 at 6:32
@Sheng be sure to look attentivly at the phase diagram to see where the supercritical fluid rests, look at phase diagrams in the Wikipedia and SklogWiki. Phase diagramm is 2D and "above a point" is not a good definition of a place on a 2D plane. Personally I remember studing phase diagrams several times (in various coodrdinates) and each time I discovered something new. Yes, that may sound stupid --- how such a simple thing as a phase diagram may suprise you. And once I didn't know about supercritical fluid either, though I was told about it, I just missed it somehow. – Yrogirg Dec 25 '12 at 6:51
"The other thing I would be concerned of is the coexistence of vapor and liquid in the simulation. If it takes place everything is wrong." Well, very near the critical point the local phase can vary rapidly on the basis of fluxuations resulting in an effective mix of phases... Not that it chalanages the core claim. – dmckee Dec 25 '12 at 19:19

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