Ab initio perturbed ion calculations are reported for neutral stoichiometric (MgO)_n (n<13) clusters. A great
number of isomer structures are identified and studied. For the isomers of (MgO)_n (n<=7) clusters, a full
geometrical relaxation is considered. Correlation corrections are included for all cluster sizes using the
Coulomb-Hartree-Fock model proposed by Clementi [IBM J. Res. Dev. 9, 2 (1965)]. The results obtained
compare favorably with the experimental data and other previous theoretical studies. The inclusion of correlation
is crucial in order to achieve a good description of these systems. We find a number of important isomers
that allow us to interpret the experimental magic numbers without the assumption of structures based on
(MgO)_3 subunits. Finally, as an electronic property, the variations in the cluster ionization potential with the
cluster size are studied and related to the structural isomer properties.
