“…This is a prototype model of many-body correlations in an interacting fermion system, in which an electron gas of density ρ e interacts with a collection of heavy classical particles ("ions") of density ρ i through an on-site interaction of strength U . It has several interesting interpretations, including as a model of valence fluctuations in rare earth and transition metal oxides, as a version of the Hubbard model, and as a model of binary alloys (for background discussion, see Kennedy and Lieb 1986, Lieb 1986, Jȩdrzejewski, Lach and Lyżwa 1989a,b, Kennedy 1994, Watson and Lemański 1995, Gruber et al 1995, Gruber and Macris 1996. Let us focus on the particular case of the neutral model (ρ e = ρ i ) in the large-U limit, and on the question of determining the ground state ionic configurations, which are those for which the electrons' energy is a minimum, at fixed density.…”