Role of the gas-liquid phase transition in the theory of nuclear matter

A sufficient condition for the two-nucleon interaction to produce a gaseous instability in the form of the appearance of inhomogeneous-spatial-density single-particle states of lower energy than the (homogeneous) plane-wave states is found to be simply that it gives binding in first order. The critical density at which the instability occurs thus signals the unambiguous breakdown of a plane-wave-based Hartree-Fock perturbation expansion for the ground state properties. Several examples are analysed.