The intermolecular interaction between a CO2 molecule and an MgO cluster has been investigated by ab initio molecular orbital method in order to test a selection rule for chemical modifications of surfaces. Favorable adsorption geometry has been determined by partial geometry optimizations initiated from three possible formations of the molecular complex. A CO3- form where the carbon atom is coordinated to an O atom of the MgO cluster is found to be most stable. The Kitaura-Morokuma energy decomposition analysis has been carried out to investigate the nature of the intermolecular interaction between a CO2 molecule and an MgO cluster. It is found that charge transfer energy from the MgO cluster to CO2 is the dominant component in the total stabilization energy. Two Mg atoms are replaced by two Ca atoms to enhance the electron flow from an MgO cluster to a CO2 molecule, which resulted in the increase in the adsorption energy. The effect of the chemical modification is examined by the energy decomposition analysis. We confirm that the increase of the cohesive energy is a consequence of the increase in charge transfer interaction from the cluster to the CO2 molecule. The mechanism of the charge transfer is also studied by the molecular orbital analysis.
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