278 research outputs found
Assessment of the TCA functional in computational chemistry and solid-state physics
We assess the Tognetti-Cortona-Adamo (TCA) generalized gradient approximation
correlation functional [J. Chem. Phys. 128:034101 (2008)] for a variety of
electronic systems. We find that, even if the TCA functional is not exact for
the uniform electron gas, it is very accurate for the jellium surface
correlation energies and it gives a realistic description of the quantum
oscillations and surface effects of various jellium clusters, that are
important model systems in computational chemistry and solid-state physics.
When the TCA correlation is combined with the non-empirical PBEint, Wu-Cohen,
and PBEsol exchange functionals, the resulting exchange-correlation
approximations provide good performances for a broad palette of systems and
properties, being reasonably accurate for thermochemistry and geometry of
molecules, transition metal complexes, non-covalent interactions,equilibrium
lattice constants, bulk moduli, and cohesive energies of solids.Comment: 14 pages, 6 figure
Semiclassical atom theory applied to solid-state physics
Using the semiclassical neutral atom theory, we extend to fourth order the
modified gradient expansion of the exchange energy of density functional
theory. This expansion can be applied both to large atoms and solid-state
problems. Moreover, we show that it can be employed to construct a simple and
non-empirical generalized gradient approximation (GGA) exchange-correlation
functional competitive with state-of-the-art GGAs for solids, but also
reasonably accurate for large atoms and ordinary chemistry.Comment: 10 pages, 7 figure
Ground state properties of heavy alkali halides
We extend previous work on alkali halides by calculations for the heavy-atom
species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic
effects are included by means of energy-consistent pseudopotentials,
correlations are treated at the coupled-cluster level. A striking deficiency of
the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from
experimental values which is reduced to a maximum error of 2.4 % by taking into
account electron correlation. Besides, we provide ab-initio data for in-crystal
polarizabilities and van der Waals coefficients.Comment: accepted by Phys. Rev.
Cohesive properties of alkali halides
We calculate cohesive properties of LiF, NaF, KF, LiCl, NaCl, and KCl with
ab-initio quantum chemical methods. The coupled-cluster approach is used to
correct the Hartree-Fock crystal results for correlations and to systematically
improve cohesive energies, lattice constants and bulk moduli. After inclusion
of correlations, we recover 95-98 % of the total cohesive energies. The lattice
constants deviate from experiment by at most 1.1 %, bulk moduli by at most 8 %.
We also find good agreement for spectroscopic properties of the corresponding
diatomic molecules.Comment: LaTeX, 10 pages, 1 figure, accepted by Phys. Rev.
Global hybrids from the semiclassical atom theory satisfying the local density linear response
We propose global hybrid approximations of the exchange-correlation (XC)
energy functional which reproduce well the modified fourth-order gradient
expansion of the exchange energy in the semiclassical limit of many-electron
neutral atoms and recover the full local density approximation (LDA) linear
response. These XC functionals represent the hybrid versions of the APBE
functional [Phys. Rev. Lett. 106, 186406, (2011)] yet employing an additional
correlation functional which uses the localization concept of the correlation
energy density to improve the compatibility with the Hartree-Fock exchange as
well as the coupling-constant-resolved XC potential energy. Broad energetical
and structural testings, including thermochemistry and geometry, transition
metal complexes, non-covalent interactions, gold clusters and small
gold-molecule interfaces, as well as an analysis of the hybrid parameters, show
that our construction is quite robust. In particular, our testing shows that
the resulting hybrid, including 20\% of Hartree-Fock exchange and named hAPBE,
performs remarkably well for a broad palette of systems and properties, being
generally better than popular hybrids (PBE0 and B3LYP). Semi-empirical
dispersion corrections are also provided.Comment: 12 pages, 4 figure
Density-functional embedding using a plane-wave basis
The constrained electron density method of embedding a Kohn-Sham system in a
substrate system (first described by P. Cortona, Phys. Rev. B {\bf 44}, 8454
(1991) and T.A. Wesolowski and A. Warshel, J. Phys. Chem {\bf 97}, 8050 (1993))
is applied with a plane-wave basis and both local and non-local
pseudopotentials. This method divides the electron density of the system into
substrate and embedded electron densities, the sum of which is the electron
density of the system of interest. Coupling between the substrate and embedded
systems is achieved via approximate kinetic energy functionals. Bulk aluminium
is examined as a test case for which there is a strong interaction between the
substrate and embedded systems. A number of approximations to the
kinetic-energy functional, both semi-local and non-local, are investigated. It
is found that Kohn-Sham results can be well reproduced using a non-local
kinetic energy functional, with the total energy accurate to better than 0.1 eV
per atom and good agreement between the electron densities.Comment: 11 pages, 4 figure
Experiments on the influence of intake conditions on local instantaneous heat flux in reciprocating internal combustion engines
[EN] The present study tries to be a contribution for the development of more precise theoretical models for predicting the dissipation of heat through the combustion chamber walls of reciprocating (internal combustion) IC engines. A fast response thermocouple was embedded in the combustion chamber of a single cylinder engine to measure instantaneous wall temperatures. The heat flux was obtained by solving the one-dimensional transient energy equation with transient boundary conditions using the Fast Fourier Transform. The engine was tested under different operating conditions to evaluate the sensitivity of the measurement procedure to variations of three relevant combustion parameters: injection pressure, air temperature and oxygen concentration at the intake. The local heat flux obtained was compared with other relevant parameters that characterize the thermal behaviour of engines, showing, in most of the cases, correlation among them. The results showed that the instantaneous heat flux through the walls and hence the local wall temperatures are strongly affected by the ignition delay and the start of combustion. © 2010 Elsevier Ltd.Desantes, J.; Torregrosa, AJ.; Broatch, A.; Olmeda González, PC. (2011). Experiments on the influence of intake conditions on local instantaneous heat flux in reciprocating internal combustion engines. Energy. 36(1):60-69. doi:10.1016/j.energy.2010.11.011S606936
In situ redox reactions facilitate the assembly of a mixed-valence metal-organic nanocapsule
C-alkylpyrogallol[4]arenes (PgCs) have been studied for their ability to form metal-organic nanocapsules (MONCs) through coordination to appropriate metal ions. Here we present the synthesis and characterization of an MnII/MnIII-seamed MONC in addition to its electrochemical and magnetic behavior. This MONC assembles from 24 manganese ions and 6 PgCs, while an additional metal ion is located on the capsule interior, anchored through the introduction of bridging nitrite ions. The latter originate from an in situ redox reaction that occurs during the self-assembly process, thus representing a new route to otherwise unobtainable nanocapsules
Study of cosmogenic activation above ground for the DarkSide-20k experiment
The activation of materials due to exposure to cosmic rays may become an important background source for experiments investigating rare event phenomena. DarkSide-20k, currently under construction at the Laboratori Nazionali del Gran Sasso, is a direct detection experiment for galactic dark matter particles, using a two-phase liquid-argon Time Projection Chamber (TPC) filled with 49.7 tonnes (active mass) of Underground Argon (UAr) depleted in 39Ar. Despite the outstanding capability of discriminating
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background in argon TPCs, this background must be considered because of induced dead time or accidental coincidences mimicking dark-matter signals and it is relevant for low-threshold electron-counting measurements. Here, the cosmogenic activity of relevant long-lived radioisotopes induced in the experiment has been estimated to set requirements and procedures during preparation of the experiment and to check that it is not dominant over primordial radioactivity; particular attention has been paid to the activation of the 120 t of UAr used in DarkSide-20k. Expected exposures above ground and production rates, either measured or calculated, have been considered in detail. From the simulated counting rates in the detector due to cosmogenic isotopes, it is concluded that activation in copper and stainless steel is not problematic. The activity of 39Ar induced during extraction, purification and transport on surface is evaluated to be 2.8% of the activity measured in UAr by DarkSide-50 experiment, which used the same underground source, and thus considered acceptable. Other isotopes in the UAr such as 37Ar and 3H are shown not to be relevant due to short half-life and assumed purification methods
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