Impacts of energetic clusters, consisting of hundreds or thousands of atoms on a solid surface, have some remarkable effects, one of which is surface smoothing. To understand its mechanism, we have examined the results of molecular dynamics (MDs) simulations of cluster collisions with a solid which revealed that cluster impacts form craters on the surface and that ejected atoms have a significant lateral momentum component (parallel to the surface). It is postulated that these energetic atoms which are not in thermal equilibrium with the surface are responsible for rapid surface diffusion and consequently the surface smoothing. A new model of surface smoothing by energetic clusters has been developed. The model is based on nonlinear dynamics of a surface profile described by the noisy Kuramoto-Sivashinsky equation. A Monte Carlo procedure is used for simulating cluster impacts by distributing on a surface craters whose volume is defined by MDs simulations. The results of calculations show a qualitative agreement with experimental data on smoothing of Cu surface with 20 kV Ar clusters of 1000 atoms.