The atomic structure of the Al-induced clusters on Si(001) surface formed by the annealing of 0.5 ML Al/Si(001) at 500 degrees C has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS results proposed that the Al atoms occupy the cave site (T4 site) and off-centered T4 site. To determine the structure of the Al-induced clusters definitely, classical ion-scattering trajectory simulations using scattering and recoiling imaging code (SARIC) have been performed for the recently proposed most possible four different cluster models (Bunk, Zotov, Kotlyar, and Zavodinsky model). Our CAICISS spectra and simulation results show that the Bunk model is the best plausible one among the models. As the results of the simulations, it is found that Al-Si dimers has been oriented on the topmost layer of the Si(001) surface with a bonding length (Delta z) of 1.00 +/- 0.05 angstrom. (c) 2005 Elsevier B.V. All rights reserved.