An approach of identifying atoms in different shells of an atomic cluster is presented, using which we have distinguished the surface atoms of the Lennard-Jones clusters containing 7-617 atoms, or termed as LJ7 to LJ617 clusters. The core structure change of the clusters has been observed, revealing that compression from the outermost surface atomic shells is responsible for the dominance of icosahedra in small clusters. (c) 2005 Elsevier B.V. All rights reserved.