Two structural parameters are quantitatively defined to describe the deformation and the surface effect of clusters. Based on these two parameters, an energetic model is formulated to describe pair potential clusters. The predicated 'phase diagram' by this model is in good agreement with previous results. Current studies also demonstrate that polytetrahedral clusters, which are only favorable at small cluster sizes, have the largest deformation energy and lowest surface energy. (C) 2009 Elsevier B. V. All rights reserved.