The Gas-Liquid Coexistence by Gibbs Ensemble Model illustrates the method introduced by Panagiotopoulos to simulate gas-liquid equilibrium. It combines NVT, NPT and ?VT Monte Carlo techniques in a single simulation using two boxes at given initial densities and no explicit interface, and setting the required temperature. When equilibrium is attained one of the boxes contains the gas, the other contains the liquid so that the chemical potentials and pressures turn equal in both boxes. In the so-called Gibbs Ensemble method there is no need to specify chemical potentials and pressures in advance, they are just natural sub-products of the simulations. The method is applied to the gas-liquid diagram of the Lennard-Jones system paving the way to the study of more complex systems. The number of molecules, initial densities and required temperature can be changed to probe the whole diagram. 3D animations of molecular moves are included. Some technical aspects concerned with finite size effects and the limitation of the method near the critical point are pointed out.

The Gas-Liquid Coexistence by Gibbs Ensemble Model was developed using the Easy Java Simulations (EJS) modeling tool.  It is distributed as a ready-to-run (compiled) Java archive.   Double clicking the jar file will run the program if Java is installed.  You can modify this simulation if you have EJS installed by right-clicking within the map and selecting "Open Ejs Model" from the pop-up menu item.

Please note that this resource requires at least version 1.7 of Java (JRE).