The ab initio molecular orbital analysis of tetrathiafulvalene (TTF) and TTF thiolate derivative adsorbed on the Au(111) surface was performed by means of the natural bond orbital (NBO) scheme to comprehend the interaction in Au-S bonds. The sulfur atoms of thiolate group have the direct connections with the Au surface, whereas those of TTF ring have the planar connections. In addition, not the 5d atomic orbitals of Au but 6s orbitals principally contribute to the interaction. (c) 2005 Elsevier B.V. All rights reserved.