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?