Uniform electron gases

We show that the traditional concept of the uniform electron gas (UEG)-a homogeneous system of finite density, consisting of an infinite number of electrons in an infinite volume-is inadequate to model the UEGs that arise in finite systems. We argue that, in general, a UEG is characterized by at least two parameters, viz. the usual one-electron density parameter ρ and a new two-electron parameter η. We outline a systematic strategy to determine a new density functional E (ρ, η) across the spectrum of possible ρ and η values.