Lyapunov instability of fluids composed of diatomic molecules

Abstract

We developed a molecular dynamics simulation method to evaluate the full spectrum of Lyapunov exponents for simple fluids and rigid diatomic fluids. And we calculated the Kolmogorov-Sinai entropy (KS entropy) and self-diffusion coefficients. We show the general trends and characteristic features of the spectra of the Lyapunov exponents, and discuss the different contributions between translational and rotational degrees of freedom depending on the density. It is found that, the KS entropy, when expressed in terms of the average collision frequency, is uniquely related to the self-diffusion coefficient by a simple scaling law for simple fluids. This behavior is also observed in diatomic molecular systems. For the three-dimensional diatomic molecular system, we can relate both translational diffusion coefficients and rotational diffusion coefficients to the respective portions of KS entropy.