I am trying to generate a phase diagram of pressure versus temperature in a NPT ensemble using molecular dynamics method. I am using the Lennard Jones potential with 108 particles and Berendsen thermostat and barostat to fix the parameter.

In order to observe phase transition I plot the radial distribution function (RDF) and also density versus temperature graph for various fixed temperature. However it is hard to differentiate liquid and gaseous phase from RDF.

Another peculiar thing is that there should be a discontinuity in density as the transition temperature is reached, which I observe only for solid liquid transition but not for the liquid vapor transition. The change in density should be huge for liquid vapor transition. Any idea on what possibly goes wrong?

The parameters:

Periodic boundary  
cut off radius = 2.5  
Truncated and shifted Lennard Jones potential  
number of particle = 108  
number of steps = 2000  
timestep = 0.004  
Berendsen barostat of scale coeefficient of 0.1 is used to scale the size of simulation box

The density versus temperature graph for pressure of 1.0MPa:

The solid liquid transition can be seen but not the liquid vapor transition.

  • $\begingroup$ I agree with Richard that with 108 particles, you're not going to see anything on the RDF. Also, you give us a pressure in units of MPa, rather than a reduced pressure; thus, we cannot check your simulations. Finally, the reduced critical pressure for LJ fluids is around 0.14; make sure you're below that, or you might simply have a supercritical fluid, as Richard (again) suggested. $\endgroup$
    – F'x
    Commented Jan 31, 2013 at 15:55

1 Answer 1


I think your transition, if present, may be drowned out by noise, as your RDF is only produced by 108 particles. As the Lennard-Jones interaction is quite simple and you are using cutoffs (good idea!), is it possible to increase the number of particles in your simulation somewhat, or else take the mean of some large number of trials?

This said, it does not take much imagination to see a continuous PV plot from about 0.7 onwards, which might mean you have something analogous to a supercritical fluid. Maybe try a number of trials at different pressures?


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