Using force fields based on quantum mechanics, we calculate the stable crystal structure for decanethiol dimers on Au(111) surfaces. We find that the optimum structure leads to an X-ray diffraction pattern identical with that recently determined by Eisenberger and co-workers. This confirms the c(4 x 2) cell determined using He and X-ray diffraction, providing the first atomistic structural description of a self-assembled monolayer (SAM). The excellent agreement between theory and experiment suggests that theory could be used to predict structures for new SAMs, allowing the design to be carried out in advance of experiment.