Interaction of water with a metal surface: Importance of van der Waals forces

Choosing the water bilayer/Rh(111) interface as an example, we study the interaction of water with a metal surface, by taking into account the van der Waals (vdW) interactions using the vdW density functional (vdW-DF). There are two types of water in a water bilayer on the substrate, namely, chemisorbed and physisorbed ones. We show that for a chemisorbed water molecule, vdW-DF results agree well with those obtained using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation. However, for a physisorbed water molecule, PBE severely underestimates the interaction energy. When we correctly include the vdW interaction, the relative strengths of the water-substrate and water-water interactions among different bilayer structures become comparable, and the water bilayer structures considered (H-up, buckled H-down, and H-flat water bilayers) become quasidegenerated, while PBE predicts one configuration more stable than the others. The structure of the water bilayer is determined by a delicate balance of Pauli repulsion and long-range vdW attractions in water-substrate and water-water interactions. Therefore, for the prediction of the water bilayer structure on the Rh(111) surface, it is important to describe Pauli repulsion and vdW interactions correctly.