The energy profile of the lowest energy path connecting the slipped parallel (C-2h) and T-shaped (C-2 upsilon) dimers of carbon dioxide was investigated using a reasonably large 6-311+ G(2df) basis set with the MP2 level electron correlation correction and with the counterpoise correction, There was no inversion barrier and the T-shaped dimer was a transition state connecting the two slipped parallel dimers. The calculated interaction energies of the two dimers (-1.290 and -1.072 kcal/ mel) were larger than those reported previously. The calculated energy difference of the two dimers (0.218 kcal/mol) was also larger than the previous values (less than 0.1 kcal/mol). The effects of basis set, electron correlation, and basis set superposition error were studied in detail. The second-order Moller-Plesset (MP2) interaction energies of the two dimers were close to the coupled cluster calculations using single and double substitutions including noniteratively triple excitations [CCSD(T)] ones. The choice of the basis set greatly affected the calculated interaction energies. Small basis sets underestimated the attractive interaction energies. Diffuse polarization functions were essential to evaluate the attractive interaction. The calculated MP2 interaction energies of the two dimers with the counterpoise correction increased by the augmentation of the diffuse polarization functions to the 6-311G* basis set, These values were close to the estimated MP2 interaction energies at the complete basis set (-1.362 and -1.140 kcal/mol), The 6-311+ G(2df) basis set was reasonably large to evaluate the attractive interactions. Although the basis set augmented with diffuse polarization functions had large basis set superposition error (BSSE), the calculated interaction energies with the counterpoise correction were close to those calculated with the nearly BSSE free cc-pVSZ and aug-cc-pVQZ basis sets and to the expected values at the complete basis set.