1,461 research outputs found
Accuracy of basis-set extrapolation schemes for DFT-RPA correlation energies in molecular calculations
We construct a reference benchmark set for atomic and molecular
random-phase-approximation (RPA) correlation energies in a density functional
theory (DFT) framework at the complete basis set limit. This set is used to
evaluate the accuracy of some popular extrapolation schemes for RPA
all-electron molecular calculations. The results indicate that for absolute
energies accurate results, clearly outperforming raw data, are achievable with
two-point extrapolation schemes based on quintuple- and sextuple-zeta basis
sets. Moreover, we show that results in good agreement with the benchmark can
also be also obtained by using a semiempirical extrapolation procedure based on
quadruple- and quintuple-zeta basis sets. Finally, we analyze the performance
of different extrapolation schemes for atomization energies.Comment: 10 pages, 2 figure
Accurate non-covalent interaction energies via an efficient MP2 scaling procedure
Using the observed proportionality of CCSD(T) and MP2 correlation interaction
energies [I. Grabowski, E. Fabiano, F. Della Sala, Phys. Chem. Chem. Phys. 15,
15485 (2013)] we propose a simple scaling procedure to compute accurate
interaction energies of non-covalent complexes. Our method makes use of MP2 and
CCSD(T) correlation energies, computed in relatively small basis sets, and
fitted scaling coefficients to yield interaction energies of almost complete
basis set limit CCSD(T) quality. Thanks to the good transferability of the
scaling coefficients involved in the calculations, good results can be easily
obtained for different intermolecular distances.Comment: 8 pages, 4 figure
Construction of a general semilocal exchange-correlation hole model: Application to nonempirical meta-GGA functionals
Using a reverse-engineering method we construct a meta-generalized gradient
approximation (meta-GGA) angle-averaged exchange-correlation hole model which
has a general applicability. It satisfies known exact hole constraints and can
exactly recover the exchange-correlation energy density of any reasonable
meta-GGA exchange-correlation energy functional satisfying a minimal set of
exact properties. The hole model is applied to several non-empirical meta-GGA
functionals: the Tao-Perdew-Staroverov-Scuseria (TPSS), the revised TPSS
(revTPSS) and the recently Balanced LOCalization (BLOC) meta-GGA of L.A.
Constantin, E. Fabiano, and F. Della Sala, (J. Chem. Theory Comput. 9, 2256
(2013)). The empirical M06-L meta-GGA functional is also considered. Real-space
analyses of atoms and ions as well as wave-vector analyses of jellium surface
energies, show that the meta-GGA hole models, in particular the BLOC one, are
very realistic and can reproduce many features of benchmark XC holes. In
addition, the BLOC hole model can be used to estimate with good accuracy the
Coulomb hole radius of small atoms and ions. Thus, the proposed meta-GGA hole
models provide a valuable tool to validate in detail existing meta-GGA
functionals, and can be further used in the development of DFT methods beyond
the semilocal level of theory.Comment: 11 pages, 10 figure
Kohn-Sham Kinetic Energy Density in the Nuclear and Asymptotic Regions: Deviations from the Von Weizs\"acker Behavior and Applications to Density Functionals
We show that the Kohn-Sham positive-definite kinetic energy (KE) density
significantly differs from the von Weizs\"acker (VW) one at the nuclear cusp as
well as in the asymptotic region. At the nuclear cusp, the VW functional is
shown to be linear and the contribution of p-type orbitals to the KE density is
theoretically derived and numerically demonstrated in the limit of infinite
nuclear charge, as well in the semiclassical limit of neutral large atoms. In
the latter case, it reaches 12 of the KE density. In the asymptotic region we
find new exact constraints for meta Generalized Gradient Approximation
(meta-GGA) exchange functionals: with an exchange enhancement factor
proportional to , where is the common meta-GGA
ingredient, both the exchange energy density and the potential are proportional
to the exact ones. In addition, this describes exactly the large-gradient limit
of quasi-two dimensional systems.Comment: 5 pages, 3 figure
Wave-function and density functional theory studies of dihydrogen complexes
We performed a benchmark study on a series of dihydrogen bond complexes and
constructed a set of reference bond distances and interaction energies. The
test set was employed to assess the performance of several wave-function
correlated and density functional theory methods. We found that second-order
correlation methods describe relatively well the dihydrogen complexes. However,
for high accuracy inclusion of triple contributions is important. On the other
hand, none of the considered density functional methods can simultaneously
yield accurate bond lengths and interaction energies. However, we found that
improved results can be obtained by the inclusion of non-local exchange
contributions.Comment: 15 pages, 7 figure
Kinetic and Exchange Energy Densities near the Nucleus
We investigate the behavior of the kinetic and the exchange energy densities
near the nuclear cusp of atomic systems. Considering hydrogenic orbitals, we
derive analytical expressions near the nucleus, for single shells, as well as
in the semiclassical limit of large non-relativistic neutral atoms. We show
that a model based on the helium iso-electronic series is very accurate, as
also confirmed by numerical calculations on real atoms up to two thousands
electrons. Based on this model, we propose non-local density-dependent
ingredients that are suitable for the description of the kinetic and exchange
energy densities in the region close to the nucleus. These non-local
ingredients are invariant under the uniform scaling of the density, and they
can be used in the construction of non-local exchange-correlation and kinetic
functionals.Comment: 11 pages, 7 figure
Meta-GGA exchange-correlation functional with a balanced treatment of nonlocality
We construct a meta-generalized-gradient approximation which properly
balances the nonlocality contributions to the exchange and correlation at the
semilocal level. This non-empirical functional shows good accuracy for a broad
palette of properties (thermochemistry, structural properties) and systems
(molecules, metal clusters, surfaces and bulk solids). The accuracy for several
well known problems in electronic structure calculations, such as the bending
potential of the silver trimer and the dimensional crossover of anionic gold
clusters, is also demonstrated. The inclusion of empirical dispersion
corrections is finally discussed and analyzed.Comment: 10 pages, 4 figure
Effects of macroscopic-polarization built-in electrostatic fields in III-V nitrides multi-quantum-wells
Huge built-in electric fields have been predicted to exist in wurtzite III-V
nitrides thin films and multilayers. Such fields originate from heterointerface
discontinuities of the macroscopic bulk polarization of the nitrides. Here we
discuss the background theory, the role of spontaneous polarization in this
context, and the practical implications of built-in polarization fields in
nitride nanostructures. To support our arguments, we present detailed
self-consistent tight-binding simulations of typical nitride QW structures in
which polarization effects are dominant.Comment: RevTeX 10 pages, 9 figure
Gradient-dependent upper bound for the exchange-correlation energy and application to density functional theory
We propose a simple gradient-dependent bound for the exchange-correlation
energy (sLL), based on the recent non-local bound derived by Lewin and Lieb. We
show that sLL is equivalent to the original Lieb-Oxford bound in
rapidly-varying density cases but it is tighter for slowly-varying density
systems. To show the utility of the sLL bound we apply it to the construction
of simple semilocal and non-local exchange and correlation functionals. In both
cases improved results, with respect to the use of Lieb-Oxford bound, are
obtained showing the power of the sLL bound.Comment: 5 pages, 2 figure
Laplacian-level kinetic energy approximations based on the fourth-order gradient expansion: Global assessment and application to the subsystem formulation of density functional theory
We test Laplacian-level meta-generalized gradient approximation (meta-GGA)
non-interacting kinetic energy functionals based on the fourth-order gradient
expansion (GE4). We consider several well known Laplacian-level meta-GGAs from
literature (bare GE4, modified GE4, and the MGGA functional of Perdew and
Constantin [Phys. Rev. B \textbf{75},155109 (2007)]), as well as two newly
designed Laplacian-level kinetic energy functionals (named L0.4 and L0.6).
First, a general assessment of the different functionals is performed, testing
them for model systems (one-electron densities, Hooke's atom and different
jellium systems), atomic and molecular kinetic energies as well as for their
behavior with respect to density-scaling transformations. Finally, we assess,
for the first time, the performance of the different functionals for Subsystem
Density Functional Theory (DFT) calculations on non-covalently interacting
systems. We find that the different Laplacian-level meta-GGA kinetic
functionals may improve the description of different properties of electronic
systems but no clear overall advantage is found over the best GGA functionals.
Concerning Subsystem DFT calculations, the here proposed L0.4 kinetic energy
functional is competitive with state-of-the-art GGAs, whereas all other
Laplacian-level functionals fail badly. The performance of the Laplacian-level
functionals is rationalized thanks to a two-dimensional reduced-gradient and
reduced-Laplacian decomposition of the non-additive kinetic energy density.Comment: 19 pages, 6 figure
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