The structure and adsorption of Mo27S((54-x)) (x = 1-6) clusters have been investigated using density functional theory method. It was found that considerable relaxation occurred at the (10 10) edge. The activity has been analyzed on the basis of frontier molecular orbital properties. The results suggest that the possible catalytic sites might be situated between two vicinal 4-fold coordinative unsaturated site CUS. The formation of Mo27S((54-x)) (x = 4-6) from Mo27S54 is easy in the presence of atomic hydrogen, but difficult under molecular hydrogen. But the situation is different from the case of Mo27S((54-x)) formation (x = 1-3) that is facile under both the atomic and molecular hydrogen reagent (under the normal FfYD/RDS condition). The adsorption of thiophene at various vacancies on the (10 10) edge of MoS2 represents that thiophene is unstable to flatly adsorb at the vacancy Of Mo27S50. Although uprightly adsorbed thiophene is adsorbed weakly, the flatly adsorbed thiohene is strongly activated when adsorbed between two vicinal 4-fold CUS (9, 10). On the basis of Hirshfeld charge analysis, the donation and back-donation between the thiophene and the substrate might account for the activation of thiophene. (c) 2005 Elsevier B.V. All rights reserved.