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 present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K ͑M 1 ͒; and 1 alkali, alkaline earth or 3d / 4d transition metal atom ͑M 2 ͒ plus two to five ͑BH 4 ͒ − groups, i.e., M 1 M 2 ͑BH 4 ͒ 2-5 , using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M 1 ͑Al/ Mn/ Fe͒͑BH 4 ͒ 4 , ͑Li/ Na͒Zn͑BH 4 ͒ 3 , and ͑Na/ K͒͑Ni/ Co͒͑BH 4 ͒ 3 alloys are found to be the most promising, followed by selected M 1 ͑Nb/ Rh͒͑BH 4 ͒ 4 alloys.

One of the standard generalized-gradient approximations (GGAs) in use in modern electronicstructure theory, PBE, and a recently proposed modification designed specifically for solids, PBEsol, are identified as particular members of a family of functionals taking their parameters from different properties of homogeneous or inhomogeneous electron liquids. Three further members of this family are constructed and tested, together with the original PBE and PBEsol, for atoms, molecules and solids. We find that PBE, in spite of its popularity in solid-state physics and quantum chemistry, is not always the best performing member of the family, and that PBEsol, in spite of having been constructed specifically for solids, is not the best for solids. The performance of GGAs for finite systems is found to sensitively depend on the choice of constraints steaming from infinite systems. Guidelines both for users and for developers of density functionals emerge directly from this work.PACS numbers: 71.15. Mb,71.10.Ca,31.15.eg Modern density-functional theory (DFT) 1,2,3 owes its success and popularity largely to the availability of simple and reliable density functionals.4 Among the most widely used such functionals are gradient-dependent approximations, such as the B88 exchange functional 5 or the PBE generalized-gradient approximation (GGA) for exchange and correlation.6 Although many other functionals are available, PBE is today the de facto standard for gradient-dependent functionals in solid-state physics, and, together with B88, in quantum chemistry. These gradient-dependent functionals also form the basis for the development of more sophisticated functionals, of, e.g. the meta-GGA or hybrid type.Given the importance of PBE both for countless practical applications of DFT, as well as for constructing more refined approximations, it is not surprising that over the years many variations of the basic PBE form have been developed. 7,8,9,10,11 Most of these are more empirical than the original construction, in the sense that they include parameters fitted to test sets of selected systems and properties. None is uniformly better than the original PBE for all systems and properties. For more restricted classes of systems, however, it is not that hard to improve on PBE, as is illustrated by the recent proposal of PBEsol, 12 which was designed to improve on PBE specifically for solids. As this is still an extraordinary large and diverse class of systems, even an improvement 'only' for solids is still a very major step forward, and consequently PBEsol is currently being implemented in many standard electronic-structure codes, and intensely scrutinized. 13,14In the present paper we point out that the step that led from PBE to PBEsol is, in fact, not unique, and allows several variations. We propose a family of functionals, which we call PBE(β,µ), of which both the original PBE and the original PBEsol are particular members, and which includes at least three more alternatives. Each member of this family takes the value of the β an...

Eleven density functionals are compared with regard to their performance for the lattice constants of solids. We consider standard functionals, such as the local-density approximation and the Perdew-Burke-Ernzerhof (PBE) generalized-gradient approximation (GGA), as well as variations of PBE GGA, such as PBEsol and similar functionals, PBE-type functionals employing a tighter Lieb-Oxford bound, and combinations thereof. Several of these variations are proposed here for the first time. On a test set of 60 solids we perform a system-by-system analysis for selected functionals and a full statistical analysis for all of them. The impact of restoring the gradient expansion and of tightening the Lieb-Oxford bound is discussed, and confronted with previous results obtained from other codes, functionals or test sets. No functional is uniformly good for all investigated systems, but surprisingly, and pleasingly, the simplest possible modifications to PBE turn out to have the most beneficial effect on its performance. The atomization energy of molecules was also considered and on a testing set of six molecules, we found that the PBE functional is clearly the best, the others leading to strong overbinding.

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.