The vapor liquid equilibria for Ar (1) + Ar_dimer (2) system were calculated by molecular dynamics simulation to test the athermal terms of activity coefficient models. The calculations were carried out at 141 K. The interatomic distance between two atoms in Ar_dimer molecule was fixed to the distance of the first peak of the radial distribution function of pure Ar at 141 K. Lennard-Jones potential was adopted for the interatomic potentials between Ar-Ar, Ar-atom in Ar_dimer, and atom in Ar_dimer-atom in Ar dimer. The number of atoms used for the calculations were from 3000 to 4200. The calculated results of activity coefficients for Ar (1) + Ar_dimer (2) system at 141 K are smaller than unity, and are between the calculated results by Flory-Huggins equation based on hard-core volume and that based on volume. (C) 2015 Elsevier B.V. All rights reserved.