A mechanism for growth on the diamond (110) surface, with dicarbon (C-2) as the growth species, is examined. Reaction energies and activation energies-of the various steps in the mechanism were investigated on model systems using molecular quantum mechanics, including the AM1 semiempirical method and the BLYP/63 1G* density functional method. The BLYP/6-31G* method yielded reaction energies and activation barriers in reasonable agreement with the results of G2 theory on some simple, related reactions. Two models for a hydrogen-terminated diamond (110) surface were employed, one with 18 carbon atoms (C18H26) and another with 46 carbon atoms (C46H50) The results indicate that C-2 addition to diamond (110) is highly exothermic with small activation barriers (< 5 kcal/mol). Insertion of C-2 into CH bonds on the model surface to form an ethylene-like adsorbate is energetically favorable, resulting in energy lowerings of 150-180 kcal per mole of C-2. Formation of single bonds between adjacent adsorbed C-2 units can be initiated by the addition of a hydrogen atom to one of the adsorbed, ethylene-like C-2 moieties. The linking of two C-2 units by this process is exothermic. The formation of single bonds between adjacent adsorbed C-2 units can also occur directly, without initiation by hydrogen addition, and is exothermic for the linking of three or more C-2 units. By either pathway the formation of a C-C single bond on the surface is exothermic by 40-50 kcal/mol.