Novel lattice-searching method for modeling the optimal strain-free close-packed isomers of clusters.

Small icosahedral, decahedral, and fcc structures have been studied by unbiased global optimization methods or Wulff construction and Northby lattice methods. Strain-free close-packed structures are not much discussed because the structures are very difficult to optimize and there is no common strain-free close-packed lattice. We propose a new strategy to construct such a lattice containing all possible strain-free close-packed isomers, and by searching the lattice with an efficient method the optimal close-packed structures were modeled. Testing with the Morse potential at rho0=14.0 for cluster size 10<or=N<or=250 showed that optimal strain-free close-packed (scp) structures are lower in energy than fcc structures in most cases even for the well-known magic numbers of fcc structures (e.g., N=201). It was found that, due to the gaps in next-nearest-neighbor contacts, fcc will become energetically incomparable with scp at very large clusters with a pair potential. Moreover, compared with the results in the literature, some new global minima for Morse clusters at large rho0 values are given.