where σ σ is an effective particle diameter and Ω Ω ( T ) is a function of temperature whose explicit form depends on the interparticle interaction law. For rigid elastic spheres, Ω Ω ( T ) is independent of T and very close to 1 . More complex interaction laws introduce a weak temperature dependence. The precise nature of the dependence is not always easy to discern, however, as Ω Ω ( T ) is defined as a multi-dimensional integral which may not be expressible in terms of elementary functions, but must be evaluated numerically. However, for particles interacting through a Mie potential (a generalisation of the Lennard-Jones potential) highly accurate correlations for Ω Ω ( T ) in terms of reduced units have been developed.