Influence of mass difference on dynamic properties of isotope mixtures

We present the results of molecular dynamics computer simulations of a two-component isotope mixture of Lennard-Jones particles, monodisperse in size but different in masses, at a fixed average density and temperature. We study changes in properties that result from mass heterogeneity, by calculating the diffusion coefficient, velocity autocorrelation function, non-Gaussian and isotope effect parameters, as functions of the degree of mass difference. Our results show that eventhough static properties are not influenced by variation in mass, the dynamic properties are significantly affected and exhibit the presence of “characteristic” values in the mass difference. We also demonstrate that our model gives a simple counter example to a recently proposed universal scaling law for atomic diffusion in condensed matter (Nature 381 (1996) 137).