The potential-energy curves of the ground and low-lying excited states for Mg+NH3 along the N-H distance were examined by the ab initio configuration interaction method. The photoinduced hydrogen elimination reaction found by the recent experiment is considered to occur via the ground-state channel. The geometries, energetics, and electronic nature of the ground-state Mg+(NH3)(n) and MgNH2+(NH3)(n-1) (n=1-6) were also investigated by second-order Moller-Plesset perturbation theory and compared with those of the corresponding hydrated species. In contrast to Mg+(H2O)(n), the successive solvation energies of Mg+(NH3)(n) become as large as those of MgNH2+(NH3)(n-1) containing the Mg2+-NH2- core for n=5 and 6, because of the growing one-center ion-pair state with the Mg2+ and the diffuse solvated electron. As a result, the solvation energies of the MgNH2+(NH3)(n-1) are insufficient to overcome the huge endothermicity of Mg+(NH3)-->MgNH2++H, even at these sizes, which is responsible for no observation of the H-loss products, MgNH2+(NH3)(n-1). (C) 2004 American Institute of Physics.