A new molecular dynamics force field for Nafion (R) containing Li+ ions has been generated using Density Functional Theory calculations (B3LYP) on a Nafion side-chain, a Li+ ion and a H2O molecule. The depth of the potential energy well between Li+ and the sulphonate group was decreased with similar to 10 kcal/mol and the optimal Li-S distance 0.5 angstrom shorter, as compared to force fields generated without water present. Molecular dynamics simulations based on the new force field result in a self-diffusion coefficient for Li+ of 8.0 x 10(-8) cm(2)/s at 353 K, which is closer to experimental result than previous simulations using force fields based on pure Nation-cation interactions. (C) 2009 Elsevier Ltd. All rights reserved.