Ab initio study of ion-ammonia complexes: Geometry and many-body interactions

Ion-ammonia complexes, M(NH₃)_n, where M=Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺ and Al³⁺, and n=1, 2, 3, 4 and 6, were studied using quantum chemical calculations at the Hartree-Fock, density functional (B3LYP) and MP2 levels. The geometry, binding energies and many-body interactions were studied. It was found that many-body interactions for M(NH₃)₆ amount to 26, 20, 14, 42, 29, 19 and 41% of the pair interaction energy, where M=Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺ and Al³⁺, respectively. Although B3LYP is in good agreement with MP2 in predicting geometry, it overestimates the interaction energies. Electron correlation based on the MP2 approximation was found to be insignificant in calculating the percent error of the pair potential.