Isotope effect of diffusion in a simple liquid

The diffusional isotope effect of a monatomic Lennard-Jones liquid is calcu lated by molecular-dynamics simulation. Mass differences of 10-40% were use d. At equilibrium density, with decreasing temperature a strong reduction o f the isotope effect is found that indicates a marked increase of the colle ctivity of motion. Changing the density at constant temperature, the same e ffect is seen that shows that the density change is the main driving force behind the reduction of the isotope effect.