Gas\index{gas} and liquids ([ph:physt:Diu89]) correspond to a random repartition of elementary bricks (atoms of molecules). Gas particles are characterized by a large kinetic energy with respect to the typical interaction energy between molecules. When kinetic energy becomes of the same order than typical interaction energy between molecules, gas transforms into liquid. Van der Walls model allows to model the liquid vapor phase transition (see section secvanderwaals).