Finite Size Effects and Cooperativity in a Model Diatomic Supercooled Liquid

We use large scale molecular dynamics simulations to investigate the relation between cooperativity and size effects in a simple diatomic supercooled liquid. We find that below a characteristic temperature the transport properties depend on the size of the simulation box. This effect then increases upon decreasing the temperature. For the model molecules constituting our liquid, the smaller box corresponds to the larger diffusion coefficient. Thus, the size effects increase the diffusion of the liquid with our molecules in opposition with previous results concerning silica and atomic soft molecules. As a result, the temperature dependence of the diffusion coefficient tends to an Arrhenius law for smaller boxes in our simulations. This result is in agreement with a cooperativity origin of non-Arrhenius behavior and size effects in fragile supercooled liquids, as a small box cutoff cooperative motions thus inducing a constant activation energy. To investigate that picture further we then study the size dependence of cooperative motions in our model liquid.