Correlated relativistic all-electron supermolecular ab initio calculations of the ground state potential of the Ar-I-2 molecule are presented. The role of differential intramonomer spin-orbit and correlation effects in the interaction energy is investigated and found to be only of minor importance. Two energetically very similar minima of the Ar-I-2 complex are found, corresponding to a linear and a T-shaped geometry of the monomers. The comparatively large isomerization barrier for the two conformations indicates the existence of two stable isomers at very low temperatures.