Structures and stabilities of doubly charged (MgO)nMg2+ (n=1-29) cluster ions

Ab initio perturbed ion plus polarization calculations are reported for doubly charged nonstoichiometric (MgO)(n)Mg2+ (n = 1 - 29) cluster ions. We consider a large number of isomers with full relaxations of the geometries, and add the correlation correction to the Hartree-Fock energies for all cluster sizes. The polarization contribution is included at a semiempirical level also for all cluster sizes. Comparison is made with theoretical results for neutral (MgO) n clusters and singly charged alkali-halide cluster ions. Our method is also compared to phenomenological pair potential models in order to assess their reliability for calculations on small ionic systems. The large coordination-dependent polarizabilities of oxide anions favor the formation of surface sites, and thus bulk-like structures begin to dominate only after n = 24. The relative stabilities of the cluster ions against evaporation of an MgO molecule show variations that are in excellent agreement with the experimental abundance spectra. (C) 1999 American Institute of Physics. [S0021-9606(99)30610-3].