We performed molecular dynamics simulations of the soft- and the hardsphere simple systems, respectively, for the number densities ranging from 0.5 to 1.0, and calculated the Kolmogorov-Sinai entropy (KS entropy) and self-diffusion coefficents. It is found that, the KS entropy, when expresseded in terms of the average collision frequency, is uniquely related to the self-diffusion coefficient by a simple scaling law. Also the behavior of the KS entropy depending on the average collision frequency and the number density is explored. The numerical results show that the scaling laws proposed by Dzugutov, and by Beijeren, DorfIman， Posch, and Dellago, respectively, can be applied to both the soft- and the hard-sphere systems by changing to more generalized forms. Next we developed a molecular dynamics method to evaluate the full Lyapunov spectrum for two dimensional fluids composed of diatomic molecules. The general trends and characteristic features of the Lyapunov spectra and the KS entropy, depending on the anisotropy dependent density and the difference of mass of two atoms, are examined for the short-range repulsive potentials. We also found that, from the analysis of the mean-squared X-components of the tangent vectors, the major contributions to the instability of the phase-space trajectory come from the momentum variables for both the simple systems and the diatomic systems.
