The step-by-step hydrogen-terminated (SSHT) model is proposed as a model for the surfaces of metal oxides. Using this model, it is found that the vibronic coupling density (VCD) can be employed as a reactivity index for surface reactions. As an example, the regioselectivity of CO2 adsorption on the Ag-loaded Ga2O3 photo-catalyst surface is investigated based on VCD analysis. The cluster model constructed by the SSHT approach reasonably reflects the electronic structures of the Ga2O3 surface. The geometry of CO2 adsorbed on the Ag-loaded Ga2O3 cluster has a bent structure, which is favorable for its photocatalytic reduction to CO.