The adsorption of the thiophene-2-thiolate and thiophen-2-yl-methanethiolate radicals has been investigated on the Au(111) surface using density functional theory under the framework of the generalized gradient approximation for the exchange-correlation functionals. In order to underscore the quantum size effects on the adsorption geometry, the Au(111) surface was modeled using a finite-sized cluster (Au-3 and Au-24) truncated from the surface as well as a periodic slab consisting of 100 atoms. The results reveal that the preferential adsorption site differs for the cluster models and slab approaches. The directional nature of the Au-S bond and the influence of the back bond of the terminal sulfur atom are found to play key roles in the adsorption geometry. The adsorption energies suggest that the binding energies for the cluster models are stronger than the slab. Inclusion of an alkyl group in between the thiophene ring and the thiol group enhances the interaction energies of the gold-sulfur bonds. (C) 2003 American Institute of Physics.