The molecular dynamics simulation method was employed to study the mechanism of silicon (001) surface smoothing by impact of Ar16 or Ar40 clusters with energy at or below 20 eV per constituent atom. Smoothing of a pyramid on top of an otherwise “flat” silicon surface was used as a model system to elucidate the mechanism of cluster-substrate interaction. Surface smoothing is achieved by lateral displacement of substrate atoms during cluster impact. There exists an optimum energy of around 4–5 eV per constituent atom of the cluster for efficient surface smoothing; this implies that a proper energy is required for effective lateral displacement. Cluster size also affects surface smoothing because lateral displacement depends on the nonlinear effect of multiple collisions in the near surface region. As anticipated, damage in the substrate increases with cluster energy. © 1999 American Institute of Physics.