Electronic and atomic structure of copper clusters

The electronic and geometrical structure of Cu microclusters is investigated utilizing a parametrized tight-binding linear-muffin-tin-orbital (TB-LMTO) method in combination with the real-space recursion technique. The parameters of the TB-LMTO Hamiltonian are derived from ab initio self-consistent k-space TB-LMTO calculations for the corresponding cluster material in its bulk phase for varying lattice constants. It is found that Cu clusters exhibit certain similarities to simple-metal clusters with respect to the gross electronic features: The calculated density-of-state (DOS) spectra exhibit exceptionally large gaps for sizes, corresponding to the well-known magic numbers of the spherical jellium model. Likewise, this shell structure shows up as pronounced drops at the magic sizes in calculated ionization potentials, in agreement with recently published experimental data. For the geometrical structure of Cu clusters we find high-symmetry arrangements to be Jahn-Teller unstable. The geometry of the magic ${\mathrm{Cu}}_{20}$ cluster, obtained by means of the simulated annealing strategy, exhibits a high sphericity, but low overall symmetry. The calculated root-mean-square bond-length fluctuation in ${\mathrm{Cu}}_{20}$ qualitatively matches the results from molecular dynamics simulations for other materials. Due to the dramatically growing computational difficulties with increasing cluster size, for larger clusters with sizes N>55, only uniformly relaxed fcc structures were examined. For such geometries, the DOS spectra in the range N≊500 are found to be almost fully converged to the corresponding k-space result of solid Cu.