1,744 research outputs found
The shortcomings of semi-local and hybrid functionals: what we can learn from surface science studies
A study of the adsorption of CO on late 4d and transition metal (111)
surfaces (Ru, Rh, Pd, Ag, Os, Ir, and Pt) considering atop and hollow site
adsorption is presented. The applied functionals include the gradient corrected
PBE and BLYP functional, and the corresponding hybrid Hartree-Fock density
functionals HSE and B3LYP. We find that PBE based hybrid functionals
(specifically HSE) yield, with the exception of Pt, the correct site order on
all considered metals, but they also considerably overestimate the adsorption
energies compared to experiment. On the other hand, the semi-local BLYP
functional and the corresponding hybrid functional B3LYP yield very
satisfactory adsorption energies and the correct adsorption site for all
surfaces. We are thus faced with a Procrustean problem: the B3LYP and BLYP
functionals seem to be the overall best choice for describing adsorption on
metal surfaces, but they simultaneously fail to account well for the properties
of the metal, vastly overestimating the equilibrium volume and underestimating
the atomization energies. Setting out from these observations, general
conclusions are drawn on the relative merits and drawbacks of various
semi-local and hybrid functionals. The discussion includes a revised version of
the PBE functional specifically optimized for bulk properties and surface
energies (PBEsol), a revised version of the PBE functional specifically
optimized to predict accurate adsorption energies (rPBE), as well as the
aforementioned BLYP functional. We conclude that no semi-local functional is
capable to describe all aspects properly, and including non-local exchange also
only improves some, but worsens other properties.Comment: 12 pages, 6 figures; to be published in New Journal of Physic
Why is a noble metal catalytically active? The role of the O-Ag interaction in the function of silver as an oxidation catalyst
Extensive density-functional theory calculations, and taking into account
temperature and pressure, affords a comprehensive picture of the behavior and
interaction of oxygen and Ag(111), and provides valuable insight into the
function of silver as an oxidation catalyst. The obtained phase-diagram reveals
the most stable species present in a given environment and thus identifies (and
excludes) possibly active oxygen species. In particular, for the conditions of
ethylene epoxidation, a thin oxide-like structure is most stable, suggesting
that such atomic O species are actuating the catalysis, in contrast to hitherto
proposed molecular-like species.Comment: 4 pages including 3 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Controlled culture of adherent cells in a novel, closed bioreactor system for cell therapy production
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