1,944 research outputs found
Van der Waals Interactions in Density Functional Theory by combining the Quantum Harmonic Oscillator-model with Localized Wannier Functions
We present a new scheme to include the van der Waals (vdW) interactions in
approximated Density Functional Theory (DFT) by combining the Quantum Harmonic
Oscillator model with the Maximally Localized Wannier Function technique. With
respect to the recently developed DFT/vdW-WF2 method, also based on Wannier
Functions, the new approach is more general, being no longer restricted to the
case of well separated interacting fragments. Moreover, it includes higher than
pairwise energy contributions, coming from the dipole--dipole coupling among
quantum oscillators. The method is successfully applied to the popular S22
molecular database, and also to extended systems, namely graphite and H
adsorbed on the Cu(111) metal surface (in this case metal screening effects are
taken into account). The results are also compared with those obtained by other
vdW-corrected DFT schemes
Phase diagram of H2 adsorbed on graphene
The phase diagram of the first layer of H adsorbed on top of a single
graphene sheet has been calculated by means of a series of diffusion Monte
Carlo (DMC) simulations. We have found that, as in the case of He, the
ground state of molecular hydrogen is a commensurate
structure, followed, upon a pressure increase, by an incommensurate triangular
solid. A striped phase of intermediate density was also considered, and found
lying on top of the equilibrium curve separating both commensurate and
incommensurate solids.Comment: 5 pages, 3 figure
ANIMAL HEALTH: THE POTENTIAL ROLE FOR LIVESTOCK DISEASE INSURANCE
Livestock Production/Industries, Risk and Uncertainty,
Surface term for the capillary condensation transitions in a slit geometry
It is shown that a bare simple fluid model (SFM) proposed some years ago for
studying adsorption between two semi-infinite solid walls can be improved by
modifying the surface term in the grand potential for the film phase. Such a
correction substantially improves the agreement between the predictions for
phase transitions provided by that SFM and results obtained from calculations
carried out for He with the density-functional method at zero temperature.
The corrective term depends on the strength of the adsorption potential and
observables of bulk helium.Comment: 4 pages, 1 table and 5 figure
Many-body dispersion effects in the binding of adsorbates on metal surfaces
A correct description of electronic exchange and correlation effects for
molecules in contact with extended (metal) surfaces is a challenging task for
first-principles modeling. In this work we demonstrate the importance of
collective van der Waals dispersion effects beyond the pairwise approximation
for organic--inorganic systems on the example of atoms, molecules, and
nanostructures adsorbed on metals. We use the recently developed many-body
dispersion (MBD) approach in the context of density-functional theory [Phys.
Rev. Lett. 108, 236402 (2012); J. Chem. Phys. 140, 18A508 (2014)] and assess
its ability to correctly describe the binding of adsorbates on metal surfaces.
We briefly review the MBD method and highlight its similarities to
quantum-chemical approaches to electron correlation in a quasiparticle picture.
In particular, we study the binding properties of xenon,
3,4,9,10-perylene-tetracarboxylic acid (PTCDA), and a graphene sheet adsorbed
on the Ag(111) surface. Accounting for MBD effects we are able to describe
changes in the anisotropic polarizability tensor, improve the description of
adsorbate vibrations, and correctly capture the adsorbate--surface interaction
screening. Comparison to other methods and experiment reveals that inclusion of
MBD effects improves adsorption energies and geometries, by reducing the
overbinding typically found in pairwise additive dispersion-correction
approaches
Quantized vortices around wavefront nodes, 2
Quantized vortices can occur around nodal points in wavefunctions. The derivation depends only on the wavefunction being single valued, continuous, and having continuous first derivatives. Since the derivation does not depend upon the dynamical equations, the quantized vortices are expected to occur for many types of waves such as electromagnetic and acoustic. Such vortices have appeared in the calculations of the H + H2 molecular collisions and play a role in the chemical kinetics. In a companion paper, it is shown that quantized vortices occur when optical waves are internally reflected from the face of a prism or particle beams are reflected from potential energy barriers
Phases of lattice hard core bosons in a periodic superlattice
We study by Quantum Monte Carlo simulations the phase diagram of lattice hard
core bosons with nearest-neighbour repulsive interactions, in the presence of a
super-lattice of adsorption sites. For a moderate adsorption strength, the
system forms crystal phases registered with the adsorption lattice; a
"supersolid" phase exists, on both the vacancy and interstitial sides, whereas
at commensuration the superfluid density vanishes. The possible relevance of
these results to experiments on He films adsorbed on graphite is discussed.Comment: 5 pages, 5 figure
Electronic transport in a Cantor stub waveguide network
We investigate theoretically, the character of electronic eigenstates and
transmission properties of a one dimensional array of stubs with Cantor
geometry. Within the framework of real space re-normalization group (RSRG) and
transfer matrix methods we analyze the resonant transmission and extended
wave-functions in a Cantor array of stubs, which lack translational order.
Apart from resonant states with high transmittance we unravel a whole family of
wave-functions supported by such an array clamped between two-infinite ordered
leads, which have an extended character in the RSRG scheme, but, for such
states the transmission coefficient across the lead-sample-lead structure
decays following a power-law as the system grows in size. This feature is
explained from renormalization group ideas and may lead to the possibility of
trapping of electronic, optical or acoustic waves in such hierarchical
geometries
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