An analytic representation of the short-range repulsion energy in ionic systems is described that allows for the fact that ions may change their size and shape depending on their environment. This function is extremely efficient to evaluate relative to previous methods of modeling the same physical effects. Using a well-defined parametrization procedure we have obtained parameter sets for this energy function that reproduce closely the density functional theory potential energy surface of bulk MgO. We show how excellent agreement can be obtained with experimental measurements of phonon frequencies and temperature and pressure dependences of the density by using this effective potential in conjunction with ab initio parametrization. (C) 2003 American Institute of Physics.