A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof ͑PBE͒ ͓Phys. Rev. Lett. 77, 3865 ͑1996͔͒. Subsequently Zhang and Yang ͓Phys. Rev. Lett. 80, 890 ͑1998͔͒ have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional ͑revPBE͒ also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni͑100͒, Ni͑111͒, Rh͑100͒, Pd͑100͒, and Pd͑111͒ surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion locally. ͓S0163-1829͑99͒02711-3͔

A grid-based real-space implementation of the projector augmented wave ͑PAW͒ method of Blöchl ͓Phys. Rev. B 50, 17953 ͑1994͔͒ for density functional theory ͑DFT͒ calculations is presented. The use of uniform three-dimensional ͑3D͒ real-space grids for representing wave functions, densities, and potentials allows for flexible boundary conditions, efficient multigrid algorithms for solving Poisson and Kohn-Sham equations, and efficient parallelization using simple real-space domain-decomposition. We use the PAW method to perform all-electron calculations in the frozen core approximation, with smooth valence wave functions that can be represented on relatively coarse grids. We demonstrate the accuracy of the method by calculating the atomization energies of 20 small molecules, and the bulk modulus and lattice constants of bulk aluminum. We show that the approach in terms of computational efficiency is comparable to standard plane-wave methods, but the memory requirements are higher.

No abstract

No abstract

Gradient-corrected density-functional theory ͑DFT-GGA͒ periodic slab calculations have been used to analyze the binding of atomic hydrogen on monometallic Pd͑111͒, Re͑0001͒, and bimetallic Pd ML /Re(0001) ͓pseudomorphic monolayer of Pd͑111͒ on Re͑0001͔͒ and Re ML /Pd(111) surfaces. The computed binding energies of atomic hydrogen adsorbed in the fcc hollow site, at 100% surface coverage, on the Pd͑111͒, Re͑0001͒, Pd ML /Re(0001), and Re ML /Pd(111) surfaces, are Ϫ2.66, Ϫ2.82, Ϫ2.25, and Ϫ2.78 eV, respectively. Formal chemisorption theory was used to correlate the predicted binding energy with the location of the d-band center of the bare metal surfaces, using a model developed by Hammer and Nørskov. The DFTcomputed adsorption energies were also analyzed on the basis of the density of states ͑DOS͒ at the Fermi level for the clean metal surfaces. The results indicate a clear correlation between the d-band center of the surface metal atoms and the hydrogen chemisorption energy. The further the d-band center is from the Fermi level, the weaker is the chemisorption bond of atomic hydrogen on the surface. Although the DOS at the Fermi level may be related to the location of the d-band, it does not appear to provide an independent parameter for assessing surface reactivity. The weak chemisorption of hydrogen on the Pd ML /Re(0001) surface relates to substantial lowering of the d-band center of Pd, when it is pseudomorphically deposited as a monolayer on a Re substrate.

The main aim of this paper is to investigate the market penetration and share of different wind turbine concepts during the years 1998–2002, a period when the increase in the wind power capacity is starting to mark an abrupt evolution (more than two GW per year). A detailed overview is performed based on suppliers market data and concept evaluation for each individual wind turbine type sold by the Top Ten suppliers over the selected five years. The investigation is processing information on a total number of approximately 90 wind turbine types from 13 different manufacturers, which have been on the Top Ten list of wind turbine suppliers during 1998 to 2002. The analysis is based on very comprehensive data, which cover approximately 76% of the accumulated world wind power installed at the end of 2002. The paper also provides an overall perspective on the contemporary wind turbine concepts, classified with respect to both their speed control ability and to their power control type. Trends for wind turbine concepts are discussed.

The aim of this article is to investigate the market penetration and share of different wind turbine concepts during the years 1995–2004, a period that represents the maturational era of the modern wind power industry. A detailed overview is given based on suppliers' market data and concept evaluation for each individual wind turbine type sold by the Top Ten suppliers over the selected decade. The investigation is processing information on approximately 160 wind turbine types from 22 different manufacturers that have featured in the Top Ten list of wind turbine suppliers during 1995–2004. The analysis is based on comprehensive data covering approximately 97% of the cumulative wind power installed worldwide at the end of 2004. The article also provides an overall perspective on contemporary wind turbine concepts, classified with respect to both their speed control ability and power control type. Current and future trends for wind turbine concepts are discussed. Copyright © 2006 John Wiley &Sons, Ltd.

Gradient corrected periodic density functional theory ͑DFT-GGA͒ slab calculations were used to examine the chemisorption of atomic hydrogen on various Pd-Re alloyed overlayers and uniformly alloyed surfaces. Adsorption was examined at 33% surface coverage, where atomic hydrogen preferred the three-fold fcc sites. The binding energy of atomic hydrogen is observed to vary by as much as 0.7 eV due to Pd-Re interactions. The computed adsorption energies were found to be between Ϫ2.35 eV ͓for monolayer Pd-on-Re, i.e., Pd ML /Re͑0001͔͒ and Ϫ3.05 eV ͓for Pd 33 Re 66 /Pd͑111͔͒. A d-band weighting scheme was developed to extend the Hammer-Nørskov surface reactivity model ͓Surf. Sci. 343, 211 ͑1995͔͒ to the analysis of bimetallic Pd-Re alloyed systems. The hydrogen chemisorption energies are correlated linearly to the surface d-band center, which is weighted appropriately by the d-band coupling matrix elements for Pd and Re. The farther the weighted d-band center is shifted below the Fermi energy, the weaker is the interaction of atomic hydrogen with the alloyed Pd-Re surface.