  We propose a structural path for the icosahedra <-> fcc transition in
clusters and demonstrate the transition in Pb_13 by ab initio
molecular-dynamics simulation. The proposed path can be described by using only
two variables. The energy surface on this two-dimensional space for Pb_13 was
calculated and a barrierless fcc-to-ico energy path was found. The atomic
displacements of the proposed structural transition for ico and fcc Pb_13 were
identified as one of the vibrational eigenmodes of the clusters with a soft
mode for fcc Pb_13. These agree with the energy curvatures around the two
structures, i.e. the ico Pb_13 is at the bottom of a valley on the energy
surface while the fcc Pb_13 is at a saddle point. The barriers of this
transition for larger clusters of Pb_n (n=55, 147 and 309) were also
calculated, by ab initio elastic-band method, and found being smaller than the
room-temperature thermal energy.
