Adsorption of thiophene on the coordinately unsaturated Mo atom on the (3 0 (3) over bar 0) plane of an Mo16S32 cluster has been investigated to develop fundamental understanding of the adsorption sites of MoS2, catalysts in the hydrotreatment process. By using the density functional theory (I)FIF) method, full geometry optimization and vibrational analysis of the thiophene/cluster complex have been carried out. Adsorption energies and vibrational frequencies for different adsorption configurations have been computed. The thiophene molecule remains almost flat in the upright configuration, but becomes bent in the parallel configurations. The C-S distances become longer for all the adsorption configurations, indicating that activation of the C-S bond occurs. The C=C and C-C distances become shorter and longer for the upright configuration, respectively. For the parallel configurations, the change of distances is in the opposite direction. The most stable configuration is the bridged and rotated parallel geometry. It is easy to distinguish whether thiophene is adsorbed in the upright or parallel coordination on sulfided Mo catalysts by means of the vibrational frequencies of adsorbed thiophene. With respect to the calculated vibrational frequencies of free thiophene, the upsilon(C=C)asym and upsilon(C=C)sym bands shift to higher frequencies for the upright configuration, whereas they shift to lower for the parallel configurations. (C) 2002 Elsevier Science B.V. All rights reserved.