On the importance of 5d orbitals for covalent bonding in ytterbium clusters

Ytterbium clusters with three to seven atoms have been studied by means of high-level quantum chemical ab initio calculations using energy-consistent relativistic large- and medium-core pseudopotentials, core-polarization potentials, a coupled-cluster electron correlation treatment, and large valence basis sets up to g functions. We have determined equilibrium structures, cohesive energies, vertical ionization potentials, and electron affinities, as well as vibrational frequencies for clusters with up to six atoms. To demonstrate graphically the transition from van der Waals to covalent interactions with increasing cluster size, the electron localization function (ELF) has been calculated from the Hartree-Fock wave functions for the equilibrium structures obtained from the correlated calculations. Yb clusters behave rather differently from those of group 12, though both have similar equilibrium structures, i.e., they exhibit significantly larger cohesive energies and more pronounced covalent contributions to bonding. We attribute these differences mainly to the presence of low-lying empty d orbitals in Yb, whose role in chemical bonding is qualitatively analyzed.