120 research outputs found
Characterization of Thin Film Materials using SCAN meta-GGA, an Accurate Nonempirical Density Functional
We discuss self-consistently obtained ground-state electronic properties of
monolayers of graphene and a number of beyond graphene compounds, including
films of transition-metal dichalcogenides (TMDs), using the recently proposed
strongly constrained and appropriately normed (SCAN) meta-generalized gradient
approximation (meta-GGA) to the density functional theory. The SCAN meta-GGA
results are compared with those based on the local density approximation (LDA)
as well as the generalized gradient approximation (GGA). As expected, the GGA
yields expanded lattices and softened bonds in relation to the LDA, but the
SCAN meta-GGA systematically improves the agreement with experiment. Our study
suggests the efficacy of the SCAN functional for accurate modeling of
electronic structures of layered materials in high-throughput calculations more
generally
Fractional charge perspective on the band-gap in density-functional theory
The calculation of the band-gap by density-functional theory (DFT) methods is
examined by considering the behavior of the energy as a function of number of
electrons. It is found that the incorrect band-gap prediction with most
approximate functionals originates mainly from errors in describing systems
with fractional charges. Formulas for the energy derivatives with respect to
number of electrons are derived which clarify the role of optimized effective
potentials in prediction of the band-gap. Calculations with a recent functional
that has much improved behavior for fractional charges give a good prediction
of the energy gap and also for finite systems.
Our results indicate it is possible, within DFT, to have a functional whose
eigenvalues or derivatives accurately predict the band-gap
Generalized gradient approximation for solids and their surfaces
Successful modern generalized gradient approximations (GGA) are biased toward
atomic energies. Restoration of the first-principles gradient expansion for the
exchange energy over a wide range of density gradients eliminates this bias. We
introduce PBEsol, a revised Perdew-Burke-Ernzerhof GGA that improves
equilibrium properties for many densely-packed solids and their surfaces.Comment: 4pages, 2figures,2table
Kinetic energy density functionals from the Airy gas, with an application to the atomization kinetic energies of molecules
We construct and study several semilocal density functional approximations
for the positive Kohn-Sham kinetic energy density. These functionals fit the
kinetic energy density of the Airy gas and they can be accurate for integrated
kinetic energies of atoms, molecules, jellium clusters and jellium surfaces. We
find that these functionals are the most accurate ones for atomization kinetic
energies of molecules and for fragmentation of jellium clusters. We also report
that local and semilocal kinetic energy functionals can show "binding" when the
density of a spin unrestricted Kohn-Sham calculation is used.Comment: 7 pages, 7 figure
Exchange and Correlation in Open Systems of Fluctuating Electron Number
While the exact total energy of a separated open system varies linearly as a
function of average electron number between adjacent integers, the energy
predicted by semi-local density functional approximations curves upward and the
exact-exchange-only or Hartree-Fock energy downward. As a result, semi-local
density functionals fail for separated open systems of fluctuating electron
number, as in stretched molecular ions A and in solid transition metal
oxides. We develop an exact-exchange theory and an exchange-hole sum rule that
explain these failures and we propose a way to correct them via a local hybrid
functional.Comment: 4 pages, 2 figure
Assessing the Performance of Recent Density Functionals for Bulk Solids
We assess the performance of recent density functionals for the
exchange-correlation energy of a nonmolecular solid, by applying accurate
calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid
metals and non-metals. The functionals tested are the modified
Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the
second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For
completeness, we also test more-standard functionals: the local density
approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria
(TPSS) meta-GGA. We find that the recent density functionals for solids reach a
high accuracy for bulk properties (lattice constant and bulk modulus). For the
cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is
actually better for the alkali metals and alkali halides. For fair comparison
of calculated and experimental results, we consider the zero-point phonon and
finite-temperature effects ignored by many workers. We show how Gaussian basis
sets and inaccurate experimental reference data may affect the rating of the
quality of the functionals. The results show that PBEsol and AM05 perform
somewhat differently from each other for alkali metal, alkaline earth metal and
alkali halide crystals (where the maximum value of the reduced density gradient
is about 2), but perform very similarly for most of the other solids (where it
is often about 1). Our explanation for this is consistent with the importance
of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil
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