Aspects of bonding in small gold clusters

A DFT-level study on neutral or charged Aun clusters at relativistic and non-relativistic (R/NR) levels confirms the previously suggested relativistic origin of the planar structures. In addition, a number of potentially new aspects are observed: (1) The symmetries with a large relativistic contribution to the orbital interaction term, ΔERorb, are those, already spanned by the NR 6s orbitals, but now with strong 6s-5d hybridization. (2) All planar structures have an 'off-plane σ' (aka δ) bonding orbital. (3) Many molecular orbitals can be seen as linear combinations of the a1 orbitals of the smallest triangular units.