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

My problem comes when I notice that there are some states which should be in the vapor phase but exhibit the radial distribution function of a liquid (showing a broadened peak). I surmise that this may be due to the gas being at too high a density, especially above the critical point.

Hence is there any way to differentiate the liquid and vapor phase?

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Can you elaborate on where the computational portions of this question are? This sounds like a question more appropriate for chemistry or possibly physics as currently stated. – Godric Seer Dec 24 '12 at 19:31
What you're describing sounds like the hallmark of a supercritical fluid, which is precisely where the liquid and vapor phases no longer can be differentiated. – Daniel Shapero Dec 27 '12 at 8:08
As somebody who does MD, I agree this is more suited for the Chemistry board, as this is a question related more to the notion of phases than anything to do with computational science. – aeismail Dec 28 '12 at 4:38

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