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Structural and electronic properties of atomic and molecular clusters are
focus of an ever-increasing number of theoretical and experimental studies.
The issues include different stable and metastable isomeric geometries,
binding energies, relative stabilities, gap energies between highest occupied
and lowest unoccupied orbitals (HOMO-LUMO), ionization potentials etc. Basically I am interested to study these properties of transition/noble metal
clusters. In this regard, first we studied ground state structures and
cohesive energies of small Cu_n clusters using the full potential muffin-tin
orbitals (FP-LMTO) based molecular dynamics. But the problem with these kind
of ab initio calculations is they are very much computationaly expensive
and therefor we can't study larger clusters. But our main aim is to study larger clusters.
Therefore, in our next paper, we proposed a tight-binding molecular dynamics with parameters
fitted to the ab initio calculations on the small clusters and with an environment
correction, to be a powerful scheme for studying large transition/noble metal clusters, where
the tight-binding Hamiltonian reduces the computational cost dramatically. Using this
TB scheme, we studied copper clusters containing n=3-55 atoms. Firstly, before we go over
the large clusters, we have shown that this TB scheme is very efficient for studing large
clusters, by the regorous comparison with the available ab initio and experimental
results for small clusters with 3-9 atoms. We found, in the size range n=10-55, most of the
clusters adopt icosahedral structur which can be derived from the 13-atom icosahedron,
the poly-icosahedral 19-, 23-, and 26-atom clusters and the 55 atom icosahedron.
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,
where the electronic effects, electronic shell closing and electron pairing in the HOMO,
dominates over the geometrical effect to determine the relative stability.
More result could be found in Physical Review A, 69, 043203 (2004) .
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