Both Si9H12 and Si15H16 cluster models for the Si(100) surface were studied using the Hartree-Fock molecular-orbital method and density functional theory. Our investigation shows that the ground state of the Si(100) surface consists of buckled dimers, contrary to the results of a number of previous embedded cluster calculations, but in agreement with some recent slab calculations. The relaxation constraints used in previous embedded duster studies probably do not model the relaxation of the Si(100) surface layers adequately. (C) 1997 American Institute of Physics.