[en] Using state of the art methods of quantum chemistry, potential energy surfaces for the formation of CO₂(¹Σ_g⁺) and CO₂ (³B₂) from CO+O (¹D) and CO+O (³P), respectively, have been studied. At the MRSDCI level, we show that the formation of CO₂(¹Σ_g⁺) from O (³P) is strongly connected with the height of the barrier localized on the CO+O (³P) entrance channel. At the CCSD(T) level with a large basis set we calculate this barrier to be 5.9kcal/mol. Consequently, we confirm that the gas-phase formation of CO₂ in interstellar molecular clouds is inefficient. To mimic the formation of CO₂, through the Eley-Rideal mechanism, on the water ice surfaces of interstellar grains, we have extended our study to consider the formation of CO₂ in the presence of water molecules. We show, using density functional and CCSD(T) methods, that the barrier located on the CO+O (³P) reaction entrance channel is hardly affected by the presence of water molecules. We therefore suggest that CO₂ formation, through the Eley-Rideal mechanism, on the water ice surfaces of interstellar grains, should be inefficient too