In this paper we present a DFT investigation on the structure and energetics of (√3 × 2√3)R30° high coverage SAMs of n-alkanethiols up to twelve carbon atoms on a clean Au(111) surface. The surface was found to be more reconstructed for longer chains, although a surface reconstruction plateau was reached for decanethiol SAMs. The geometry was suggested to be dictated by steric effects and van der Waals interactions between the adsorbates resulting in a specific adsorption configuration. Furthermore, the increasing adsorption energy per thiol molecule is due to the increasing van der Waals forces between the adsorbates. In conclusion a more stable monolayer was obtained with increasing adsorbate chain length. In other words a subtle balance between interchain interactions and surface reconstruction is at the origin of the alkylthiol/Au(111) self-assembled monolayer geometry.