In an ideal gas the particles are assumed to interact only through collisions. The equipartition theorem may also be used to derive the energy and pressure of "non-ideal gases" in which the particles also interact with one another through conservative forces whose potential U (r) depends only on the distance r between the particles. This situation can be described by first restricting attention to a single gas particle, and approximating the rest of the gas by a spherically symmetric distribution. It is then customary to introduce a radial distribution function g (r) such that the probability density of finding another particle at a distance r from the given particle is equal to 4 πr ρg (r), where ρ = N / V is the mean density of the gas. It follows that the mean potential energy associated to the interaction of the given particle with the rest of the gas is