580 research outputs found
New approaches for incorporating the exact exchange energy density into density functional approximations
In the last ten years, hybrid density functional approximations have become the most widely used method in modern quantum chemistry. Hybrid functionals combine the semi-local exchange-correlation and a fraction of the exact-exchange energy. The most common are global hybrid functionals, with a constant fraction of the exact exchange determined emprirically. Recently, two complementary strategies have been proposed to improve the performance of hybrid functionals. In range-separated hybrid functionals, the fraction of exact exchange depends on the interelectronic distance. In local hybrid functionals, the fraction of exact exchange is position-dependent. In this work, we propose two approaches that combine range-separated and local hybrid functionals together, providing a promising route to more accurate results.
Most previous implementations of range-separated hybrid functionals use a universal, system-independent screening parameter, fitted to experimental data. However, the screening parameter proves to depend strongly on the choice of the training set. Moreover, such functionals violate the exact high-density limit. In this work, we argue that the separation between short-range (SR) and long-range (LR) interactions should depend on the local density. We propose an approximation that uses a position-dependent screening function o( r) defining a local range separation (LRS) for mixing exact (HF-type) and LSDA exchange. This method adds a substantial flexibility to describe diverse chemical compounds. Moreover, the new model satisfies a high-density limit better than the approximation with fixed screening parameter.
We have also developed an alternative strategy to improve the range-separated functionals by combining them together with local hybrid functionals. We consider two limiting cases: screened local hybrids with short-range exact exchange, and long-range corrected hybrids with long-range exact exchange. The former approach can treat metals and narrow-gap semiconductors much more efficiently than standard local hybrids do. The latter method provides the correct asymptotic behavior, which is important for the treatment of charge transfer and Rydberg excitations in finite systems
"Narrow" Graphene Nanoribbons Made Easier by Partial Hydrogenation
It is a challenge to synthesize graphene nanoribbons (GNRs) with narrow
widths and smooth edges in large scale. Our first principles study on the
hydrogenation of GNRs shows that the hydrogenation starts from the edges of
GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the
number of carbon-carbon - bonds. Furthermore, the partially
hydrogenated wide GNRs have similar electronic and magnetic properties as those
of narrow GNRs. Therefore, it is not necessary to directly produce narrow GNRs
for realistic applications because partial hydrogenation could make wide GNRs
"narrower"
Hybrid functional study of proper and improper multiferroics
We present a detailed study of the structural, electronic, magnetic and
ferroelectric properties of prototypical \textit{proper} and \textit{improper}
multiferroic (MF) systems such as BiFeO and orthorhombic HoMnO,
respectively, within density functional theory (DFT) and using the
Heyd-Scuseria-Ernzerhof hybrid functional (HSE). By comparing our results with
available experimental data as well as with state-of-the-art GW calculations,
we show that the HSE formalism is able to account well for the relevant
properties of these compounds and it emerges as an accurate tool for predictive
first-principles investigations on multiferroic systems. We show that effects
beyond local and semilocal DFT approaches (as provided by HSE) are necessary
for a realistic description of MFs. For the electric polarization, a decrease
is found for MFs with magnetically-induced ferroelectricity, such as HoMnO,
where the calculated polarization changes from 6 using
Perdew-Burke-Ernzerhof (PBE) to 2 using HSE. However, for
\textit{proper} MFs, such as BiFeO, the polarization slightly increases
upon introduction of exact exchange. Our findings therefore suggest that a
general trend for the HSE correction to bare density functional cannot be
extracted; rather, a specific investigation has to be carried out on each
compound.Comment: Revised version. In press in PCC
LiGaOS is a Fast Li-Ion Conductor: a First-Principles Prediction
Solid Li-ion conducting electrolytes are highly sought for all solid-state Li-batteries, which are considered the next-generation safe batteries. Here a systematic computational study on the intrinsic transport properties of lithium gallium oxysulfide, LiGaOS (S. G. Pmc21), as a potential solid-state Li-ion electrolyte have been reported. The phonon dispersion spectrum analysis indicates that LiGaOS crystal structure is dynamically stable. The energy band structure and density of states calculations suggest that LiGaOS is an insulator with a wide indirect band gap of ∼5.44 eV. The CI-NEB calculations reveal that the “kick-off” collective migration via Li-interstitials is the dominant conduction mechanism for Li-transport, with an extremely low energy barrier of 0.05 eV. The corresponding Li-ion self-diffusion coefficient estimated by transition state theory is in the order of magnitude of 10−3 cm2/s at room temperature, with the dominant charge carrier being Li-interstitial. Overall, the presented first-principles calculations suggest that LiGaOS is a promising solid Li-ion electrolyte for future all-solid-state lithium batteries
Development of Self-Assembling Mixed Protein Micelles with Temperature-Modulated Avidities
Elastin-like polypeptides (ELPs) are polypentapeptides that undergo hydrophobic collapse and aggregation above a specific transition temperature, Tt. ELP diblocks sharing a common “core” block (I60) but varying “outer” blocks (A80, P40) were designed, where Tt,I \u3c Tt,A \u3c Tt,P. The formation of ~55 nm diameter mixed micelles from these ELP diblocks was verified using dynamic light scattering (DLS), multiangle light scattering (MALS) and fluorescence resonance energy transfer (FRET). To confer affinity to the blood circulating protein fibrinogen, a fibrinogen-binding tetrapeptide sequence (GPRP) was fused to A80-I60, while P40-I60 was fused to a non-binding control (GPSP). The self-assembling, peptide-displaying, mixed micelles exhibit temperature-modulated avidities for immobilized and soluble fibrinogen at 32 °C and 42 °C. In this initial proof-of-concept design, the engineered mixed micelles were shown to disengage fibrinogen at elevated temperatures. The modular nature of this system can be used for developing in vivo depot systems that will only be triggered to release in situ upon specific stimuli
Quasiparticle interfacial level alignment of highly hybridized frontier levels: HO on TiO(110)
Knowledge of the frontier levels' alignment prior to photo-irradiation is
necessary to achieve a complete quantitative description of HO
photocatalysis on TiO(110). Although HO on rutile TiO(110) has been
thoroughly studied both experimentally and theoretically, a quantitative value
for the energy of the highest HO occupied levels is still lacking. For
experiment, this is due to the HO levels being obscured by hybridization
with TiO(110) levels in the difference spectra obtained via ultraviolet
photoemission spectroscopy (UPS). For theory, this is due to inherent
difficulties in properly describing many-body effects at the
HO-TiO(110) interface. Using the projected density of states (DOS) from
state-of-the-art quasiparticle (QP) , we disentangle the adsorbate and
surface contributions to the complex UPS spectra of HO on TiO(110). We
perform this separation as a function of HO coverage and dissociation on
stoichiometric and reduced surfaces. Due to hybridization with the TiO(110)
surface, the HO 3a and 1b levels are broadened into several peaks
between 5 and 1 eV below the TiO(110) valence band maximum (VBM). These
peaks have both intermolecular and interfacial bonding and antibonding
character. We find the highest occupied levels of HO adsorbed intact and
dissociated on stoichiometric TiO(110) are 1.1 and 0.9 eV below the VBM. We
also find a similar energy of 1.1 eV for the highest occupied levels of HO
when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In
both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than
those estimated from UPS difference spectra, which are inconclusive in this
energy region. Finally, we apply self-consistent QP (scQP1) to obtain
the ionization potential of the HO-TiO(110) interface.Comment: 12 pages, 12 figures, 1 tabl
Vacancy defect configurations in the metal-organic framework UiO-66: Energetics and electronic structure
Vacancy lattice sites in the metal-organic framework UiO-66 are known to have a profound effect on the material properties. Here we use density functional theory to compare the energies of defect arrangements containing missing linkers and missing metal clusters for different choices of charge compensation. Our results show that the preference for missing metal clusters or missing linker defects depends on the charge compensation as well as the overall concentration of defects in the crystal. Both regimes can be experimentally accessible depending on the synthesis conditions. We investigate the electronic structure of the different types of defects, showing that, despite some changes in the localisation of the frontier orbitals, the electronic energy levels are only weakly affected by the presence of point defects
A germanate transparent conductive oxide
Wide bandgap conductors such as In2O3 and ZnO are used as transparent conducting oxides (TCOs). To date, TCOs are realized using post transition metal cations with largely spread s-orbitals such as In3+, Sn4+, Zn2+ and Cd2+. On the other hand, no good electronic conductor has been realized in oxides of Al, Si and Ge. Here we report the conversion of an oxide of Ge into a good electronic conductor by employing the concept of superdegeneracy. We find that cubic SrGeO3, synthesized under high pressure, displays a direct bandgap of 3.5 eV, a carrier mobility of 12 cm2(Vs)−1, and conductivities of 3 Scm−1 (DC) and 400 Scm−1 (optical conductivity). This is the first Ge-based electronic conductive oxide, and expands the family of TCOs from ionic oxides to covalent oxides
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