Ab initio calculations are performed for $ArCO_2^+$ and $ArCO_2$. Between the two configurations of $ArCO_2^+$ the orbital interactions and the higher order correlation calculations favor the T-shape, and their interaction energies are calculated to be approximately half the experimental values using 6-31G$^{ast}$ basis set. In $ArCO_2$, the calculations qualitatively favor the T-structure, which is compatible with the experiment. However, the true interaction energy is obscured since it is within the BSSE limit at this basis set size and the correlation level. Addition of sp type diffuse functions increase the interaction energies by a considerable amount, but the BSSE estimated by CP method are responsible for the significant portion of the difference. The possible equilibrium structure of the $Ar^+-CO_2$ complex, where the charge is localized on Ar, is suggested as having a linear structure. The potential energy surface and the amount of charge transfer are shown to be sensitive to the type and balancing of basis set.