Structure and stability of copper clusters: A tight-binding molecular dynamics study

In this paper we propose a tight-binding molecular dynamics with parameters fitted to first-principles calculations on the smaller clusters and with an environment correction, to be a powerful technique for studying large transition-metal/noble-metal clusters. In particular, the structure and stability of Cun clusters for n=3-55 are studied by using this technique. The results for small Cun clusters (n=3-9) show good agreement with ab initio calculations and available experimental results. In the size range 10≤n≤55 most of the clusters adopt icosahedral structure which can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters, and the 55-atom icosahedron, by adding or removing atoms. However, a local geometrical change from icosahedral to decahedral structure is observed for n=40-44 and return to the icosahedral growth pattern is found at n=45 which continues. Electronic 'magic numbers' ( n=2, 8, 20, 34, 40) in this regime are correctly reproduced. Due to electron pairing in highest occupied molecular orbitals (HOMOs), even-odd alternation is found. A sudden loss of even-odd alternation in second difference of cluster binding energy, HOMO-LUMO (LUMO, lowest unoccupied molecular orbital) gap energy and ionization potential is observed in the region n∼40 due to structural change there. Interplay between electronic and geometrical structure is found