On a relationship between the collective migration of surface atoms in   microclusters and the saddle points on the potential energy surface
  Plenty of saddles on a multidimensional potential energy surface(PES) of two-dimensional microclusters, where atoms are interacting via Morse potential, are numerically located. The reaction paths emanating from the two types of the local minima, which represent the compact and the non-compact shape of Morse clusters, to their neighboring saddles on PES are elucidated. By associating the reaction path crossing these saddles with the atomic rearrangements,we evaluate the barrier height corresponding to various characteristic atomic motion accompanied by the {\it floaters} (i.e. surface atoms popped out of the cluster surface). Our findings are summarized as: (i)The saddle points implying the {\it gliding motion} of a single {\it floater} over the cluster surface yields extremely small values of the energy barriers regardless of the shapes of clusters. In particular, the {\it gliding motion} of a train composed of a few surface atoms also appears as the low-lying saddles. As a result, the barrier height corresponding to the {\it simultaneous gliding motion}, which is a manifestation of the reaction path crossing the higher-index saddles on PES, is significantly low. (ii)A surface rearrangement, where {\it floaters} are created or annihilated, implies relatively high barrier energy which is still accessible below melting point. (iii)On the other hand, the atomic motion, where atoms located deep inside of the clusters are rearranged as well as surface atoms, yields extremely high barrier energies. Some relations between these results and the recent experimental study of the surface cluster diffusion are also pointed out. 
Yasushi Shimizu, Shin-ichi Sawada, and Kensuke S.Ikeda