Strengthening of Au-Au bonds in small gold clusters by adsorbing noble gases

In state-of-the-art experiments for the vibrational spectra of metal clusters in the gas phase, photodissociation spectroscopy is performed on clusters complexed with noble gas (RG) atoms, where a RG atom is usually expected to form a weak van der Waals bond. By employing DFT (PBE functional with selected comparisons to PBE0, and to MP2 and CCSD(T) calculations), we surprisingly find a partially covalent bond of {\em neutral} dimers with RG. For RG = Ar, Kr, Xe one or two RG atoms can bind in a linear molecule with Au$_2$. While both Hirschfeld and Mulliken analyses show a small electron transfer from the RG to Au$_2$, the Au-Au bond {\em shortens} and the Au-Au stretch frequency increases. This is inconsistent with the expected effect of electron transfer to the antibonding orbital of the dimer. Electron-density ($n$) differences between the bonded systems and the isolated fragments show an accumulation of $n$ between RG and the neighboring Au atom, and between the gold atoms. The analysis of the projected density of states reveals that, although only non-bonding orbital interactions and no charge transfer occurs between RG and Au$_2$, the $d$-electrons of Au$_2$ are redistributed due to the interaction with RG in such a way that the Au-Au $\sigma_s$ bond is strengthened.