where formula_76 is an effective particle diameter and formula_77 is a function of temperature whose explicit form depends on the interparticle interaction law. For rigid elastic spheres, formula_77 is independent of formula_60 and very close to formula_66. More complex interaction laws introduce a weak temperature dependence. The precise nature of the dependence is not always easy to discern, however, as formula_77 is defined as a multi-dimensional integral which may not be expressible in terms of elementary functions. An alternate, equivalent way to present the result is in terms of the gas viscosity formula_82, which can also be calculated in the Chapman–Enskog approach: