  We use molecular dynamic simulations to investigate the relation between the
presence of packing defects in a glass-former and the spontaneous cooperative
motions called dynamic heterogeneity. For that purpose we use a simple diatomic
glass-former and add a small number of larger or smaller diatomic probes. The
diluted probes modify locally the packing, inducing structural defects in the
liquid, while we find that the number of defects is small enough not to disturb
the average structure. We find that a small packing modification around a few
molecules can deeply influence the dynamics of the whole liquid, when
supercooled. When we use small probe molecules, the dynamics accelerates and
the dynamic heterogeneity decreases. In contrast, for large probes the dynamics
slows down and the dynamic heterogeneity increases. The induced heterogeneities
and transport coefficient modification increase when the temperature decreases
and disappear around the onset temperature of the cage dynamics.
