3,581 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
Food safety
Illness induced by unsafe food is a problem of great public health significance. This study relates exclusively to the occurrence of chemical agents which will result in food unsafe for human consumption since the matter of food safety is of paramount importance in the mission and operation of the manned spacecraft program of the National Aeronautics and Space Administration
Van der Waals interactions at surfaces by DFT using Wannier functions
The method recently developed to include Van der Waals interactions in the
Density Functional Theory by using the Maximally-Localized Wannier functions,
is improved and extended to the case of atoms and fragments weakly bonded
(physisorbed) to metal and semimetal surfaces, thus opening the way to
realistic simulations of surface-physics processes, where Van der Waals
interactions play a key role. Successful applications to the case of Ar on
graphite and on the Al(100) surface, and of the H2 molecule on Al(100) are
presented.Comment: submitted to Phys. Rev. Let
Nonlinear elasticity of monolayer graphene
By combining continuum elasticity theory and tight-binding atomistic
simulations, we work out the constitutive nonlinear stress-strain relation for
graphene stretching elasticity and we calculate all the corresponding nonlinear
elastic moduli. Present results represent a robust picture on elastic behavior
of one-atom thick carbon sheets and provide the proper interpretation of recent
experiments. In particular, we discuss the physical meaning of the effective
nonlinear elastic modulus there introduced and we predict its value in good
agreement with available data. Finally, a hyperelastic softening behavior is
observed and discussed, so determining the failure properties of graphene.Comment: 4 page
Elastic properties of hydrogenated graphene
There exist three conformers of hydrogenated graphene, referred to as chair-,
boat-, or washboard-graphane. These systems have a perfect two-dimensional
periodicity mapped onto the graphene scaffold, but they are characterized by a
orbital hybridization, have different crystal symmetry, and otherwise
behave upon loading. By first principles calculations we determine their
structural and phonon properties, as well as we establish their relative
stability. Through continuum elasticity we define a simulation protocol
addressed to measure by a computer experiment their linear and nonlinear
elastic moduli and we actually compute them by first principles. We argue that
all graphane conformers respond to any arbitrarily-oriented extention with a
much smaller lateral contraction than the one calculated for graphene.
Furthermore, we provide evidence that boat-graphane has a small and negative
Poisson ratio along the armchair and zigzag principal directions of the carbon
honeycomb lattice (axially auxetic elastic behavior). Moreover, we show that
chair-graphane admits both softening and hardening hyperelasticity, depending
on the direction of applied load.Comment: submitted on Phys.Rev.
Adsorption of rare-gas atoms on Cu(111) and Pb(111) surfaces by van der Waals-corrected Density Functional Theory
The DFT/vdW-WF method, recently developed to include the Van der Waals
interactions in Density Functional Theory (DFT) using the Maximally Localized
Wannier functions, is applied to the study of the adsorption of rare-gas atoms
(Ne, Ar, Kr, and Xe) on the Cu(111) and Pb(111) surfaces, at three
high-symmetry sites. We evaluate the equilibrium binding energies and
distances, and the induced work-function changes and dipole moments. We find
that, for Ne, Ar, and Kr on the Cu(111) surface the different adsorption
configurations are characterized by very similar binding energies, while the
favored adsorption site for Xe on Cu(111) is on top of a Cu atom, in agreement
with previous theoretical calculations and experimental findings, and in common
with other close-packed metal surfaces. Instead, the favored site is always the
hollow one on the Pb(111) surface, which therefore represents an interesting
system where the investigation of high-coordination sites is possible.
Moreover, the Pb(111) substrate is subject, upon rare-gas adsorption, to a
significantly smaller change in the work function (and to a correspondingly
smaller induced dipole moment) than Cu(111). The role of the chosen reference
DFT functional and of different Van der Waals corrections, and their dependence
on different rare-gas adatoms, are also discussed
Adaptive constraints for feature tracking
In this paper extensions to an existing tracking algorithm are described.
These extensions implement adaptive tracking constraints in the form
of regional upper-bound displacements and an adaptive track smoothness
constraint. Together, these constraints make the tracking algorithm
more flexible than the original algorithm (which used fixed tracking
parameters) and provide greater confidence in the tracking results.
The result of applying the new algorithm to high-resolution ECMWF
reanalysis data is shown as an example of its effectiveness
Assessment of electron transfer mechanisms during a long-term sediment microbial fuel cell operation
The decentralized production of bioelectricity as well as the bioremediation of contaminated sediments might be achieved by the incorporation of an anode into anaerobic sediments and a cathode suspended in the water column. In this context, a sediment microbial fuel cell microcosm was carried out using different configurations of electrodes and types of materials (carbon and stainless steel). The results showed a long-term continuous production of electricity (>300 days), with a maximum voltage of approximately 100 mV reached after ~30 days of operation. A twofold increase of voltage was noticed with a twofold increase of surface area (~30 mV to ~60 mV vs. 40 cm2 to 80 cm2), while a threefold increase was obtained after the substitution of a carbon anode by one of stainless steel (~20 mV to ~65 mV vs. 40 cm2 to 812 cm2). Cyclic voltammetry was used to evaluate sediment bacteria electroactivity and to determine the kinetic parameters of redox reactions. The voltammetric results showed that redox processes were limited by the diffusion step and corresponded to a quasi-reversible electron charge transfer. These results are encouraging and give important information for the further optimization of sediment microbial fuel cell performance towards the long-term operation of sediment microbial fuel cell devices.This study was supported by the Portuguese Foundation for Science and Technology (FCT) under the scope of the strategic funding of UID/BIO/04469/2019 unit and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by the European Regional Development Fund under the scope of Norte2020 — Programa Operacional Regional doNorte. The authors also acknowledge the fellowship awarded to Gilberto Martins (SFRH/BPD/80528/2011) under the scope of the program POPH/ESF.info:eu-repo/semantics/publishedVersio
Spontaneous onset of homochirality in oligopeptide chains generated in the polymerization of N-carboxyanhydride amino acids in water
ISSN:0169-6149ISSN:1573-087
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