With ab initio molecular orbital calculations, the structures of the cation clusters Mg+(H2O)(n) and their hydrogen-eliminated products (MgOH)(+)(H2O)(n-1) are optimized. In Mg+(H2O)(n), the hydration number of the most stable isomer is 3. In (MgOH)(+)(H2O)(n-1), all water molecules are directly bonded to Mg+ for n less than or equal to 6. The hydration energy of (MgOH)(+) is larger than that of Mg+ because of the strongly polarized (MgOH)(+) molecular ion; Mg is oxidized halfway to Mg(II). The internal energy change of the hydrogen elimination of Mg+(H2O)(n) is positive for n = 1-5, but becomes negative for n = 6, which is in good agreement with the product switching in the TOF spectrum reported in the preceding paper by Sanekata et al. The effects of isotope substitution and equilibrium constants of the hydrogen (deuterium) elimination reaction observed in their experiment can be explained qualitatively.