MP2/cc-pVXZ and MP2/aug-cc-pVXZ (X = D, T Q) complete basis set extrapolation calculations were carried out on the four minimum energy structures located on the potential energy surface of the OCS . (HCCH)(2) van der Waals complex. The trimer structure consists of an averaged structure representing the dynamical interconversion between two equivalent forms with the (HCCH)(2) face adopting a T-shaped conformation. The transition structure is a energetically close-lying geometry with a near-parallel disposition of the two HCCH fragments. (C) 2004 Elsevier B.V. All rights reserved.