Elementary reaction mechanisms for adsorption and chemical bonding of C x H y fragments (molecules, radicals, clusters) to a clean, unreconstructed and fully or partly hydrogenated diamond (111) surface are studied by molecular dynamic (MD) structure investigations. Applying the Born-Oppenheimer approximation the forces moving the hydrocarbon species via MD to the bonding formation with the crystalline substrate are calculated within an alternative MD-density functional (DF) approach, which uses localized atomic orbital basis functions. In our first results using the method we obtain the stable and metastable ground state structure configurations of CH, CH 2, CH 3, C 2H, and C 2H 2 molecules and radical absorbed on a diamond (111) surface, modelled by a finite cluster. We comment on the dependence of the adsorbed cluster geometries on the surface dangling bond density and we discuss the energetically most favourable configuration in relation to low temperature, low pressure diamond growth conditions.