It is shown that ‘compressible ion’ potentials, which represent the results ofab initioelectronic structure calculations on MgO and CaO and which represent their phase transitions accurately, can be transferred successfully to other alkaline earth oxides by substitution of accepted values for the cation radii. The application of such a potential to ZnO erroneously predicts a ground-state crystal structure of rocksalt symmetry. After extending the interaction model to allow for the presence of induced octupoles on both the zinc and oxide ions the observed wurtzite structure emerges as most stable. The importance of short-range contributions to the induced moments, beyond those predicted by the multipole expansion, is highlighted.