A representation of the short-range repulsion energy in an ionic system is described which allows for the fact that an ion may be compressed by its neighbours. The total energy of the system is expressed in a pairwise additive form, but the interionic interactions have a many-body character. The form of this representation and the parameters required to represent MgO and CaO are obtained from recent ab-initio electronic structure calculations. The fact that the representation is transferable between crystals with different coordination number is demonstrated by direct comparison with nb-initio results on the different crystal types. Comparison with experimental results on the equation of state of different isomorphs and on the location of the pressure of the transition between them confirms the accuracy of the ab-initio results and of the potential derived from them in representing perfect crystal properties. A computationally efficient molecular dynamics (MD) scheme may be derived for this representation. The additional degrees of freedom which represent the varying ionic radii are constrained to their adiabatic values in the course of the simulation by an adaptation of Car and Parrinello’s method. The MD scheme is used to examine whether an ab-initio parameterized potential model which allows for the spherical compression of an oxide ion by its neighbours and for dipole polarization effects is a sufficiently good representation of the interactions in MgO to allow an accurate calculation of the phonon dispersion curves.