[en] For a simple model whose cohesion is dominated by dispersion forces we show that the expansion of the energy in terms of multi-center interactions is ill conditioned at a low density. This density is physically realizable for systems with highly polarizable atoms, and in these circumstances an alternative expression for the internal energy is required. For polarizable systems the requisite densities are readily achievable with the use of modern high pressure capabilities, and have consequences for the interpretation of equation of state data in terms of potential energy functions. 13 refs., 3 figs