This study reports second-order perturbation (MP2) theory predictions of the optimized structures and relative energies for IHI-.Ar-n (n = 1-7) complexes. For n = 1-6, the lowest energy structure has all n Ar atoms forming a partial ring in the plane that is perpendicular to and bisects the IHI- axis. The ring is closed at n = 6, and for n = 7, one of the At atoms moves into a second ring. Analysis of the geometrical parameters and three-dimensional MP2 molecular electrostatic potentials (MEP) is used to determine why the ring structure is lowest in energy for n = 1-6, but not for n = 7. Based on the MEP, it is concluded that Ar atoms tend to distribute in regions of low electron density that exist in the plane perpendicular to and bisecting the IHI- axis. Hence, the global minimum for all n < 7 is a ring structure in this plane. For n = 7, steric effects force an Ar atom into a new ring.