A similar approach was also employed in the development of a liquid-gas model [63, 38]. Only one component is present in the model and the free energy is chosen to be the free energy of a Van-der-Walls fluid [63, 65].
where
and a and b are constants determining the strength of the interaction.
With this definition of the free energy and the same definitions for the
moments of 
, see equations (4.109) and (4.110),
an equilibrium distribution function, with the same form as
equation (4.107), can be found [63, 38].