For gases whose molecules are not spherically symmetric, the expression k = f μ μ c v still holds. In contrast with spherically symmetric molecules, however, f varies significantly depending on the particular form of the interparticle interactions: this is a result of the energy exchanges between the internal and translational degrees of freedom of the molecules. An explicit treatment of this effect is difficult in the Chapman–Enskog approach. Alternately, the approximate expression f = ( 1 / 4 ) ( 9 γ γ − − 5 ) was suggested by Eucken, where γ γ is the heat capacity ratio of the gas.