it turns out that a more realistic modeling of interactions is essential for accurate prediction of the temperature dependence of formula_10, which experiments show increases more rapidly than the formula_75 trend predicted for rigid elastic spheres. indeed, the chapman–enskog analysis shows that the predicted temperature dependence can be tuned by varying the parameters in various molecular models. a simple example is the sutherland model, which describes rigid elastic spheres with "weak" mutual attraction. in such a case, the attractive force can be treated perturbatively, which leads to a particularly simple expression for formula_10: