1,555 research outputs found
Errors in Hellmann-Feynman Forces due to occupation number broadening, and how they can be corrected
In ab initio calculations of electronic structures, total energies, and
forces, it is convenient and often even necessary to employ a broadening of the
occupation numbers. If done carefully, this improves the accuracy of the
calculated electron densities and total energies and stabilizes the convergence
of the iterative approach towards self-consistency. However, such a boardening
may lead to an error in the calculation of the forces. Accurate forces are
needed for an efficient geometry optimization of polyatomic systems and for ab
initio molecular dynamics (MD) calculations. The relevance of this error and
possible ways to correct it will be discussed in this paper. The first approach
is computationally very simple and in fact exact for small MD time steps. This
is demonstrated for the example of the vibration of a carbon dimer and for the
relaxation of the top layer of the (111)-surfaces of aluminium and platinum.
The second, more general, scheme employs linear-response theory and is applied
to the calculation of the surface relaxation of Al(111). We will show that the
quadratic dependence of the forces on the broadening width enables an efficient
extrapolation to the correct result. Finally the results of these correction
methods will be compared to the forces obtained by using the smearing scheme,
which has been proposed by Methfessel and Paxton.Comment: 6 pages, 5 figures, Scheduled tentatively for the issue of Phys. Rev.
B 15 15 Dec 97 Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Adsorption of Xe atoms on metal surfaces: New insights from first-principles calculations
The adsorption of rare gases on metal surfaces serve as the paradigm of weak
adsorption where it is typically assumed that the adsorbate occupies maximally
coordinated hollow sites. Density-functional theory calculations using the
full-potential linearized augmented plane wave method for Xe adatoms on
Mg(0001), Al(111), Ti(0001), Cu(111), Pd(111), and Pt(111), show, however, that
Xe prefers low-coordination on-top sites in all cases. We identify the
importance of polarization and a site-dependent Pauli repulsion in actuating
the site preference and the principle nature of the rare-gas atom--metal
surface interaction.Comment: 5 pages including 4 figure files. Related publications can be found
at http://www.fhi-berlin.mpg.de/th/paper.htm
First-principles, atomistic thermodynamics for oxidation catalysis
Present knowledge of the function of materials is largely based on studies
(experimental and theoretical) that are performed at low temperatures and
ultra-low pressures. However, the majority of everyday applications, like e.g.
catalysis, operate at atmospheric pressures and temperatures at or higher than
300 K. Here we employ ab initio, atomistic thermodynamics to construct a phase
diagram of surface structures in the (T,p)-space from ultra-high vacuum to
technically-relevant pressures and temperatures. We emphasize the value of such
phase diagrams as well as the importance of the reaction kinetics that may be
crucial e.g. close to phase boundaries.Comment: 4 pages including 2 figure files. Submitted to Phys. Rev. Lett.
Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations
The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia
Non-Adiabatic Potential-Energy Surfaces by Constrained Density-Functional Theory
Non-adiabatic effects play an important role in many chemical processes. In
order to study the underlying non-adiabatic potential-energy surfaces (PESs),
we present a locally-constrained density-functional theory approach, which
enables us to confine electrons to sub-spaces of the Hilbert space, e.g. to
selected atoms or groups of atoms. This allows to calculate non-adiabatic PESs
for defined charge and spin states of the chosen subsystems. The capability of
the method is demonstrated by calculating non-adiabatic PESs for the scattering
of a sodium and a chlorine atom, for the interaction of a chlorine molecule
with a small metal cluster, and for the dissociation of an oxygen molecule at
the Al(111) surface.Comment: 11 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Ultrathin oxides: bulk-oxide-like model surfaces or unique films?
To better understand the electronic and chemical properties of wide-gap oxide
surfaces at the atomic scale, experimental work has focused on epitaxial films
on metal substrates. Recent findings show that these films are considerably
thinner than previously thought. This raises doubts about the transferability
of the results to surface properties of thicker films and bulk crystals. By
means of density-functional theory and approximate GW corrections for the
electronic spectra we demonstrate for three characteristic wide-gap oxides
(silica, alumina, and hafnia) the influence of the substrate and highlight
critical differences between the ultrathin films and surfaces of bulk
materials. Our results imply that monolayer-thin oxide films have rather unique
properties.Comment: 5 pages, 3 figures, accepted by PR
The steady-state of heterogeneous catalysis, studied by first-principles statistical mechanics
The turn-over frequency of the catalytic oxidation of CO at RuO2(110) was
calculated as function of temperature and partial pressures using ab initio
statistical mechanics. The underlying energetics of the gas-phase molecules,
dissociation, adsorption, surface diffusion, surface chemical reactions, and
desorption were obtained by all-electron density-functional theory. The
resulting CO2 formation rate [in the full (T, p_CO, p_O2)-space], the movies
displaying the atomic motion and reactions over times scales from picoseconds
to seconds, and the statistical analyses provide insights into the concerted
actions ruling heterogeneous catalysis and open thermodynamic systems in
general.Comment: 4 pages including 3 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Ab initio data-analytics study of carbon-dioxide activation on semiconductor oxide surfaces
The excessive emissions of carbon dioxide (CO2) into the atmosphere threaten to shift the CO2 cycle planet-wide and induce unpredictable climate changes. Using artificial intelligence (AI) trained on high-throughput first principles based data for a broad family of oxides, we develop a strategy for a rational design of catalytic materials for converting CO2 to fuels and other useful chemicals. We demonstrate that an electron transfer to the π-antibonding orbital of the adsorbed molecule and the associated bending of the initially linear molecule, previously proposed as the indicator of activation, are insufficient to account for the good catalytic performance of experimentally characterized oxide surfaces. Instead, our AI model identifies the common feature of these surfaces in the binding of a molecular O atom to a surface cation, which results in a strong elongation and therefore weakening of one molecular C-O bond. This finding suggests using the C-O bond elongation as an indicator of CO2 activation. Based on these findings, we propose a set of new promising oxide-based catalysts for CO2 conversion, and a recipe to find more
Abundant Oligonucleotides Common to Most Bacteria
BACKGROUND: Bacteria show a bias in their genomic oligonucleotide composition far beyond that dictated by G+C content. Patterns of over- and underrepresented oligonucleotides carry a phylogenetic signal and are thus diagnostic for individual species. Patterns of short oligomers have been investigated by multiple groups in large numbers of bacteria genomes. However, global distributions of the most highly overrepresented mid-sized oligomers have not been assessed across all prokaryotes to date. We surveyed overrepresented mid-length oligomers across all prokaryotes and normalised for base composition and embedded oligomers using zero and second order Markov models. PRINCIPAL FINDINGS: Here we report a presumably ancient set of oligomers conserved and overrepresented in nearly all branches of prokaryotic life, including Archaea. These oligomers are either adenine rich homopurines with one to three guanine nucleosides, or homopyridimines with one to four cytosine nucleosides. They do not show a consistent preference for coding or non-coding regions or aggregate in any coding frame, implying a role in DNA structure and as polypeptide binding sites. Structural parameters indicate these oligonucleotides to be an extreme and rigid form of B-DNA prone to forming triple stranded helices under common physiological conditions. Moreover, the narrow minor grooves of these structures are recognised by DNA binding and nucleoid associated proteins such as HU. CONCLUSION: Homopurine and homopyrimidine oligomers exhibit distinct and unusual structural features and are present at high copy number in nearly all prokaryotic lineages. This fact suggests a non-neutral role of these oligonucleotides for bacterial genome organization that has been maintained throughout evolution
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