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The structural, electronic and magnetic properties of Con clusters (n =2−20) have been investigated using density functional theory within the pseudopotential plane wave method. An unusual hexagonal growth pattern has been observed in the intermediate size range, n =15−20. The cobalt atoms are ferromagnetically ordered and the calculated magnetic moments are found to be higher than that of corresponding hcp bulk value, which are in good agreement with the recent SternGerlach experiments. The average coordination number is found to dominate over the average bond length to determine the effective hybridization and consequently the cluster magnetic moment.

We systematically investigate the structural, electronic and magnetic properties of Mnn clusters (n = 2−20) within the ab-initio pseudopotential plane wave method using generalized gradient approximation for the exchange-correlation energy. A new kind of icosahedral structural growth has been predicted in the intermediate size range. Calculated magnetic moments show an excellent agreement with the Stern-Gerlach experiment. A transition from ferromagnetic to ferrimagnetic Mn−Mn coupling takes place at n = 5 and the ferrimagnetic states continue to be the ground states for the entire size range. Possible presence of multiple isomers in the experimental beam has been argued. No signature of non-metal to metal transition is observed in this size range and the coordination dependence of d−electron localization is discussed.

In this paper we propose a tight-binding molecular dynamics with parameters fitted to firstprinciples calculations on the smaller clusters and with an environment correction, to be a powerful technique for studying large transition/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 HOMOs, even-odd alternation is found. A sudden loss of even-odd alternation in second difference of cluster binding energy, HOMO-LUMO 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.

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