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The distribution and diffusion of water with various water content in a fully crosslinked epoxy molding compound was simulated using a parallel full-atomistic molecular dynamics simulation method. We found that the free volume is 5.1%, 4.4%, and 4.0% of the total system volume at 0 wt%, 4 wt%, and 7 wt% of water content, respectively, accommodating the absorbed water molecules, where the molecules are distributed throughout the system. The hydrophilic groups of the epoxy molding compound (such as tertiary amine groups and hydroxyl groups) are uniformly distributed through the system: the average distance between the amine groups is ~9.5 ?? and that between the hydroxyl groups is 3.8-7.2 ??. The water molecules are distributed in proximity to these hydrophilic groups. By counting the number of these water molecules nearby the functional groups, we found that on average, each amine group has 2.47 and 3.86 water molecules, and each hydroxyl group has 0.61 and 0.85 water molecules at 4 wt% and 7 wt% water content, respectively. The water diffusion proceeds via the hopping mechanism and is enhanced with increasing water content: 0.1690 ?? 10-6 cm 2/s for 4 wt% water content and 0.2065 ?? 10-6 cm2/s for 7 wt% water content.