  The structures and relative stabilities of doubly-charged nonstoichiometric
(CaO)$_n$Ca$^{2+}$ (n=1--29) cluster ions and of neutral stoichiometric
(MgO)$_n$ and (CaO)$_n$ (n=3,6,9,12,15,18) clusters are studied through {\em ab
initio} Perturbed Ion plus polarization calculations. The large
coordination-dependent polarizabilities of oxide anions favor the formation of
surface sites, making the critical cluster size where anions with bulk
coordination first appear larger than that found in the related case of alkali
halides. Thus, we show that there are substantial structural differences
between alkali halide and alkaline-earth oxide cluster ions, contrary to what
is suggested by the similarities in the experimental mass spectra. An
alternative interpretation of the magic numbers for the case of oxides is
proposed, which involves an explicit consideration of isomer structures
different from the ground states. A comparison with the previously studied
(MgO)$_n$Mg$^{2+}$ cluster ions shows that the emergence of bulklike structural
properties with size is slower for calcium oxide. Nevertheless, the structures
of the doubly charged clusters are rather similar for the two materials. On the
contrary, the study of the neutrals reveals interesting structural differences
between MgO and CaO, similar to those found in the case of alkali halides.
