  In a recent Letter, Barnett, Tajima, Nishihara, Ueshima and Furukawa obtained
a theoretical expression for the maximum Lyapunov exponent $\lambda_1$ of a
dilute gas. They conclude that $\lambda_1$ is proportional to the cube root of
the self-diffusion coefficient $D$, independent of the range of the interaction
potential. They validate their conjecture with numerical data for a dense
one-component plasma, a system with long-range forces. We claim that their
result is highly non-generic. We show in the following that it does not apply
to a gas of hard spheres, neither in the dilute nor in the dense phase.
