A density functional theory (DFT) study of CO2 adsorption on barium oxide (BaO) adsorbents is conducted to understand the chemical activity of the oxygen site on the BaO (100) surface. This study evaluated the adsorption energies and geometries of a single CO2 molecule and a pair of CO2 molecules on the BaO (100) surface. A quantum calculation was performed to obtain information on the molecular structures and molecular reaction mechanisms; the results of the calculation indicated that CO2 was adsorbed on BaO to form a stable surface carbonate with strong chemisorption. To study the interactive CO2 adsorption on the BaO (100) surface, a pair of CO2 molecules was bound to neighboring and distant oxygen sites. The interactive CO2 adsorption on the BaO surface was found to slightly weaken the adsorption energy, owing to the interaction between CO2 molecules.