The force between parallel surfaces with grafted poly(ethylene oxide) chains was calculated using molecular dynamics simulations. First, a single chain surrounded by water molecules was simulated with full atomistic detail. On the basis of that, a coarse-grained model with implicit solvent was developed. The force between surfaces with grafted coarse-grained chains was calculated as a function of surface separation (3-18 nm), chain length (12-20 repeat units), grafting density (Gamma = 0.01-2.04 chains/nm(2)), and interactions between surface and polymer. Results showed that the force was mainly repulsive and increased monotonically as distance decreased. Chain length had a stronger influence on the force than Gamma. For purely repulsive interactions between polymer and surfaces, chains had extended conformations, leading to higher repulsive forces between surfaces. For surfaces with higher affinity for polymer, adsorption took place. The force between surfaces was significant only for high Gamma because chains then achieved more extended conformations.