When modeling the medium effects for the ET using molecular dynamics, it is important to account for the trade-off between computing power and simulation box size. We examine the effect of water molecule number on the Marcus parameters here. The rectangular unit cell of graphene, with size of a x = 2.469 and a y = 4.274 Å , contains four carbon atoms. To investigate the optimum number of H 2 O, a supercell with the basal area of 12.35×12.82 Å 2 (5×3 unit cell) was used. The water molecules were equally spread into the top and bottom of the graphene sheet.