The potential energy surface of the Ar-N₂ van der Waals complex is determined with high-level ab initio calculations, confirming the single well in the T-shaped configuration, predicted with empirical potentials. In the context of analytical representation of the potential, two different schemes for treating the atom-diatom interaction are compared, with the anisotropy introduced for the total molecular component and for its constituent atoms. The latter way is found to provide better predictions for the potential energy surface in terms of the same amount of input information, particularly if the latter is very limited, and thus presents a useful alternative to the traditional Legendre polynomial expansion. Comparison with empirical potentials reveals a significantly lower anisotropy of the ab initio potential. The Ar-N₂ surface obtained is used to predict the structure and stability of larger Ar_nN₂ (n > 2) clusters.