Ab initio perturbed ion plus polarization calculations are reported for doubly charged
nonstoichiometric (MgO)nMg21 (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 n524. 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
