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In order to gain insight into diamond growth, we have calculated equilibrium geometries for several adsorbates on the hydrogenated diamond <111> surface. While the adsorption height of a single methane radical onto a dangling bond is found to be in excellent agreement with the bulk-diamond bond length, the back bonded hydrogens of adjacent adsorbed methyl radicals repel one another. In contrast, adjacent acetlyinic radicals do not repel one another but lead to the introduction of double carbon bonds, misplaced carbon atoms above the active layer and a bond length which is too short in comparison to that of bulk diamond. Our calculations on the acetylene molecule near a dangling bond indicate that the resulting adsorbate bond length is substantially too large and that the carbon atom is unlikely to be stable directly above the surface carbon atom. Of the adsorbates studied, geometrical arguments suggest …