We introduce a Leonard-Jones (L-J) interaction into the two-dimensional (2D)
Collins model, and consider the existence of the holes that are called the
molecular fraction. The Gibbs free energy of solid, liquid and gas has been
derived. From the Gibbs function we have obtained the whole diagram of the 2D
monatomic L-J system, which includes the melting line, vaporization line,
sublimation line and the triple point. Also, we have discussed a few
properties of the critical point.