Before simulations of diffusivity, gas molecules, such as H2, CH4, CO2, O2 or N2 are inserted into the simulation box of the cross-linked polymer. The system is first equilibrated through a 21step MD equilibration protocol . The system is then equilibrated for 1 ns under the NVT ensemble at 300K, followed by 2 ns under the NPT ensemble at 300K and 1 atm. Production runs are then performed for a duration of 7 ns. The first 2 ns are used for equilibration and the remaining 5 ns for analysis. The diffusion coefficient of gas molecules in the cross-linked polymer is estimated by the mean squared displacement (MSD) defined as