We present a theoretical study on the mode of adsorption of the water dimer on the perfect MgO(100) surface using the ab initio embedded cluster method. Structures and normal-mode analyses were carried out at the HF level of theory, while energetic information was improved using the IMOMO methodology at the CCSD level using a smaller model system. We found that a single coadsorbed water molecule nearby can stabilize the hydroxyl species resulting from dissociation of the adsorbed water. The dissociative product is less stable by 25.5 kJ/mol compared to the molecular adsorbed water dimer. Since the reverse barrier is only 3.8 kJ/mol and is removed when zero-point energy correction is included, hydroxyl species would not be observed for the adsorption of a water dimer. Analysis on the degree of stabilization due to the coadsorbed water molecule suggests that two or more coadsorbed water molecules would yield observable hydroxyl species. These results support recent MIES experimental evidence of water dissociation on the perfect MgO(100) surface in the submonolayer regime. Our results also indicate that the effects of the Madelung potential are significant for studying chemisorption processes on ionic metal oxide crystal surfaces.