Structural, electronic, and optical properties of medium-sized Li n clusters ( n=20, 30, 40, 50) by density functional theory

Abstract

The lowest-energy structures of medium-sized Li n ( n=20, 30, 40, 50) clusters are determined from simulated annealing technique followed by geometry optimization within the framework of density functional theory. The shapes of magic-number Li 20 and Li 40 clusters are nearly spherical while those of the other clusters are ellipsoid. The growth of Li n clusters is based on core of multi-layered pentagonal bipyramids with other atoms capped on the surface. The binding energies of the Li n clusters were computed and compared with experiments. At magic-number sizes ( n=20, 40), Li n clusters possess relatively larger HOMO–LUMO gaps and higher ionization potentials, corresponding to the closure of electron shell. The molecular orbitals of the lithium clusters can be grouped into electron shells and their spatial distributions resemble the atomic orbitals. The average polarizability of the Li clusters reduces rapidly with cluster size and can be approximately described by a classical metallic sphere model. The optical absorption spectra of Li n clusters from time-dependent density functional theory calculations show giant resonance phenomenon, and resonance peak blueshifts with increasing cluster size.