We report a DFT study on the self-assembly of the fullerene derivative PCBM on the Au(111) surface. Recent STM experiments (Angew. Chem. 2007, 119, 8020-8023()) show a coverage-dependent transition of the adsorption and self-assembly of PCBM on this surface. To understand the origin of this observation, we compute the geometries and relative energies of ten PCBM dimers and four tetramers. The calculations show that the self-assembly of PCBM at high coverage is mainly controlled by hydrogen bonding between the PCBM tails. Due to the large size of the fullerene cage, the hydrogen bonds are formed far away from the surface; hence they are very similar to those found in the gas phase. This picture successfully explains the observed site-insensitive adsorption at high coverage and the 2D arrangement of PCBM on the surface.