GGA versus van der Waals density functional results for mixed gold/mercury molecules and pure Au and Hg cluster properties

Abstract

Dispersion forces, which originate the van der Waals interaction, are indispensable to describe numerous systems and processes, including metallic clusters and surfaces. In this work is used an efficient numerical implementation in the context of density functional theory of a non-local correlation van der Waals density functional (vdW-DF) to self-consistently solve the structure and electronic properties of small molecules (ArAu, AuF, ArAuF, ArCuF, Au₂Hg, Au₂Hg₂), as well as Au_2–15 and Hg_2–6clusters. Three different flavours of that vdW-DF exchange-correlation (xc) functional are tested. The results for small molecules are compared with those from the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) against experiments or highly accurate quantum chemical calculations. It is found that, on average, vdw-DF improves PBE binding energies and overestimates bond distances. Our vdW-DF calculations lead to planar structures as lowest energy isomers of Au₁₄ and Au₁₅ clusters. The calculated polarizability of Au_2–15 isomers dramatically decreases in passing from two-dimensional (2D) to three-dimensional (3D) equilibrium geometries. A combination of the density of states of two vdw-DF planar isomers of the Au₁₂⁻ anion is proposed to explain the photoelectron spectroscopy experiments. Contrary to PBE results, the vdW-DF calculations predict that the O_h isomer of Hg₆ is more stable than the C_2v one.