The hydrogen bond interaction between water molecules adsorbed on a Pd <111> surface, a nucleator of two dimensional ordered water arrays at low temperatures, is studied using density functional theory calculations. The role of the exchange and correlation density functional in the characterization of both the hydrogen bond and the water-metal interaction is analyzed in detail. The effect of non local correlations using the van der Waals density functional proposed by Dion et al. [M. Dion, H. Rydberg, E. Schröder, D. C. Langreth and B. I. Lundqvist, Phys. Rev. Lett., 2004, 92, 246401] is also studied. We conclude that the choice of this potential is critical in determining the cohesive energy of water-metal complexes. We show that the interaction between water molecules and the metal surface is as sensitive to the density functional choice as hydrogen bonds between water molecules are. The reason for this is that the two interactions are very similar in nature. We make a detailed analogy between the water-water bond in the water dimer and the water-Pd bond at the Pd <111> surface. Our results show a strong similarity between these two interactions and based on this we describe the water-Pd bond as a hydrogen bond type interaction. These results demonstrate the need to obtain an accurate and reliable representation of the hydrogen bond interaction in density functional theory.

Pure GaN is known to show a very high photocatalytic water oxidation activity in the UV range. Recently Shen et al. [J. Phys. Chem. C 2010, 114, 13695] have proposed a sequence of intermediate steps for the water oxidation process at the GaN(101̅ 0) GaN/water interface. In this contribution we show that when spontaneous water dissociation occurs, the acidity of the surface can be accurately computed using first principles molecular dynamics simulations. The electronic structure analysis of the adsorbed water and hydroxyl groups shows large differences between GaN and the well studied photocatalyst TiO 2 . On the basis of our results we argue that the search for efficient photocatalytic materials needs to take into account the water dissociation activity of candidate material surfaces.

We study the structure and dynamics of liquid water in contact with Pd and Au (111) surfaces using ab initio molecular dynamics simulations with and without van der Waals interactions. Our results show that the structure of water at the interface of these two metals is very different. For Pd, we observe the formation of two different domains of preferred orientations, with opposite net interfacial dipoles. One of these two domains has a large degree of in-plane hexagonal order. For Au, a single domain exists with no in-plane order. For both metals, the structure of liquid water at the interface is strongly dependent on the use of dispersion forces. The origin of the structural domains observed in Pd is associated to the interplay between water/water and water/metal interactions. This effect is strongly dependent on the charge transfer that occurs at the interface and which is not modeled by current state of the art semi-empirical force fields.

The geometrical and electronic structure properties of 100 and 110 silicon nanowires in the absence of surface passivation are studied by means of density-functional calculations. As we have shown in a recent publication [R. Rurali and N. Lorente, Phys. Rev. Lett. 94, 026805 (2005)] the reconstruction of facets can give rise to surface metallic states. In this work, we analyze the dependence of geometric and electronic structure features on the size of the wire and on the growth direction.