36 research outputs found
Hubbard parameters from density-functional perturbation theory
We present a transparent and computationally efficient approach for the
first-principles calculation of Hubbard parameters from linear-response theory.
This approach is based on density-functional perturbation theory and the use of
monochromatic perturbations. In addition to delivering much improved
efficiency, the present approach makes it straightforward to calculate
automatically these Hubbard parameters for any given system, with tight
numerical control on convergence and precision. The effectiveness of the method
is showcased in three case studies - CuO, NiO, and LiCoO - and by the
direct comparison with finite differences in supercell calculations.Comment: 17 pages, 6 figure
Unraveling the effects of inter-site Hubbard interactions in spinel Li-ion cathode materials
Accurate first-principles predictions of the structural, electronic,
magnetic, and electrochemical properties of cathode materials can be key in the
design of novel efficient Li-ion batteries. Spinel-type cathode materials
LiMnO and LiMnNiO are promising candidates
for Li-ion battery technologies, but they present serious challenges when it
comes to their first-principles modeling. Here, we use density-functional
theory with extended Hubbard functionals - DFT++ with on-site and
inter-site Hubbard interactions - to study the properties of these
transition-metal oxides. The Hubbard parameters are computed from
first-principles using density-functional perturbation theory. We show that
while is crucial to obtain the right trends in properties of these
materials, is essential for a quantitative description of the structural
and electronic properties, as well as the Li-intercalation voltages. This work
paves the way for reliable first-principles studies of other families of
cathode materials without relying on empirical fitting or calibration
procedures
Self-consistent Hubbard parameters from density-functional perturbation theory in the ultrasoft and projector-augmented wave formulations
The self-consistent evaluation of Hubbard parameters using linear-response
theory is crucial for quantitatively predictive calculations based on
Hubbard-corrected density-functional theory. Here, we extend a
recently-introduced approach based on density-functional perturbation theory
(DFPT) for the calculation of the on-site Hubbard to also compute the
inter-site Hubbard . DFPT allows to reduce significantly computational
costs, improve numerical accuracy, and fully automate the calculation of the
Hubbard parameters by recasting the linear response of a localized perturbation
into an array of monochromatic perturbations that can be calculated in the
primitive cell. In addition, here we generalize the entire formalism from
norm-conserving to ultrasoft and projector-augmented wave formulations, and to
metallic ground states. After benchmarking DFPT against the conventional
real-space Hubbard linear response in a supercell, we demonstrate the
effectiveness of the present extended Hubbard formulation in determining the
equilibrium crystal structure of LiMnPO (x=0,1) and the subtle
energetics of Li intercalation.Comment: 15 pages, 3 figure
Accurate electronic properties and intercalation voltages of olivine-type Li-ion cathode materials from extended Hubbard functionals
The design of novel cathode materials for Li-ion batteries would greatly
benefit from accurate first-principles predictions of structural, electronic,
and magnetic properties as well as intercalation voltages in compounds
containing transition-metal elements. For such systems, density-functional
theory (DFT) with standard (semi-)local exchange-correlation functionals is of
limited use as it often fails due to strong self-interaction errors that are
especially relevant in the partially filled shells. Here, we perform a
detailed comparative study of the phospho-olivine cathode materials
LiMnPO, LiFePO, and the mixed transition metal
LiMnFePO () using four
electronic-structure methods: DFT, DFT+, DFT++, and HSE06. We show
that DFT++, with onsite and intersite Hubbard parameters
determined from first principles and self-consistently with respect to the
structural parameters by means of density-functional perturbation theory
(linear response), provides the most accurate description of the electronic
structure of these challenging compounds. In particular, we demonstrate that
DFT++ displays very clearly "digital" changes in oxidation states of the
transition-metal ions in all compounds, including the mixed-valence phases
occurring at intermediate Li concentrations, leading to voltages in remarkable
agreement with experiments. We show that the inclusion of intersite Hubbard
interactions is essential for the accurate prediction of thermodynamic
quantities, balancing the drive for localization induced by the onsite with
intersite orbital hybridizations
Understanding the role of Hubbard corrections in the rhombohedral phase of BaTiO
We present a first-principles study of the low-temperature rhombohedral phase
of BaTiO using Hubbard-corrected density-functional theory. By employing
density-functional perturbation theory, we compute the onsite Hubbard for
Ti() states and the intersite Hubbard between Ti() and O()
states. We show that applying the onsite Hubbard correction alone to
Ti() states proves detrimental, as it suppresses the Ti()-O()
hybridization and drives the system towards a cubic phase. Conversely, when
both onsite and intersite are considered, the localized character of
the Ti() states is maintained, while also preserving the Ti()-O()
hybridization, restoring the rhombohedral phase of BaTiO. The generalized
PBEsol++ functional yields remarkable agreement with experimental results
for the band gap and dielectric constant, while the optimized geometry is
slightly less accurate compared to PBEsol. Zone-center phonon frequencies and
Raman spectra, being significantly influenced by the underlying geometry,
demonstrate better agreement with experiments in the case of PBEsol, while
PBEsol++ exhibits reduced accuracy, and the PBEsol+ Raman spectrum
diverges remarkably from experimental data, highlighting the adverse impact of
the correction alone in BaTiO. Our findings underscore the promise of
the extended Hubbard PBEsol++ functional with first-principles and
for the investigation of other ferroelectric perovskites with mixed
ionic-covalent interactions
First-principles study of the gap in the spin excitation spectrum of the CrI honeycomb ferromagnet
The nature of the gap observed at the zone border in the spin-excitation
spectrum of CrI quasi-2D single crystals is still controversial. We perform
first-principles calculations based on time-dependent density-functional
perturbation theory, which indicate that the observed gap results from a
combination of spin-orbit and inter-layer interaction effects. The former give
rise to the anisotropic spin-spin interactions that are responsible for its
very existence, while the latter determine both its displacement from the K
point of the Brillouin zone due to the in-plane lattice distortions induced by
them, and an enhancement of its magnitude, in agreement with experiments and
previous theoretical work based on a lattice model
Self-consistent site-dependent DFT+ study of stoichiometric and defective SrMnO
We propose a self-consistent site-dependent Hubbard approach for DFT+
calculations of defects in complex transition-metal oxides, using Hubbard
parameters computed via linear-response theory. The formation of a defect
locally perturbs the chemical environment of Hubbard sites in its vicinity,
resulting in different Hubbard parameters for different sites. Using oxygen
vacancies in SrMnO as a model system, we investigate the dependence of
on the chemical environment and study its influence on the structural,
electronic, and magnetic properties of defective bulk and strained thin-film
structures. Our results show that a self-consistent improves the
description of stoichiometric bulk SrMnO with respect to GGA or GGA+
calculations using an empirical . For defective systems, changes as a
function of the distance of the Hubbard site from the defect, its oxidation
state and the magnetic phase of the bulk structure. Taking into account this
dependence, in turn, affects the computed defect formation energies and the
predicted strain- and/or defect-induced magnetic phase transitions, especially
when occupied localized states appear in the band gap of the material upon
defect creation.Comment: 12 pages (+8 SI), 9 figures (+6 SI
Self-consistent DFT++ study of oxygen vacancies in SrTiO
Contradictory theoretical results for oxygen vacancies in SrTiO (STO)
were often related to the peculiar properties of STO, which is a
transition metal oxide with mixed ionic-covalent bonding. Here, we apply, for
the first time, density functional theory (DFT) within the extended Hubbard
DFT++ approach, including on-site as well as inter-site electronic
interactions, to study oxygen-deficient STO with Hubbard and parameters
computed self-consistently via density-functional perturbation theory. Our
results demonstrate that the extended Hubbard functional is a promising
approach to study defects in materials with electronic properties similar to
STO. Indeed, DFT++ provides a better description of stoichiometric STO
compared to standard DFT or DFT+, the band gap and crystal field splitting
being in good agreement with experiments. In turn, also the description of the
electronic properties of oxygen vacancies in STO is improved, with formation
energies in excellent agreement with experiments as well as results obtained
with the most frequently used hybrid functionals, however at a fraction of the
computational cost. While our results do not fully resolve the contradictory
findings reported in literature, our systematic approach leads to a deeper
understanding of their origin, which stems from different cell sizes, STO
phases, the exchange-correlation functional, and the treatment of structural
relaxations and spin-polarization.Comment: 15 pages (+11 SI), 13 figures (+6 SI
Self-consistent site-dependent DFT+ study of stoichiometric and defective SrMnO
We propose a self-consistent site-dependent Hubbard approach for DFT+
calculations of defects in complex transition-metal oxides, using Hubbard
parameters computed via linear-response theory. The formation of a defect
locally perturbs the chemical environment of Hubbard sites in its vicinity,
resulting in different Hubbard parameters for different sites. Using oxygen
vacancies in SrMnO as a model system, we investigate the dependence of
on the chemical environment and study its influence on the structural,
electronic, and magnetic properties of defective bulk and strained thin-film
structures. Our results show that a self-consistent improves the
description of stoichiometric bulk SrMnO with respect to GGA or GGA+
calculations using an empirical . For defective systems, changes as a
function of the distance of the Hubbard site from the defect, its oxidation
state and the magnetic phase of the bulk structure. Taking into account this
dependence, in turn, affects the computed defect formation energies and the
predicted strain- and/or defect-induced magnetic phase transitions, especially
when occupied localized states appear in the band gap of the material upon
defect creation.Comment: 12 pages (+8 SI), 9 figures (+6 SI
Pulay forces in density-functional theory with extended Hubbard functionals: From nonorthogonalized to orthogonalized manifolds
We present a derivation of the exact expression for Pulay forces in
density-functional theory calculations augmented with extended Hubbard
functionals, and arising from the use of orthogonalized atomic orbitals as
projectors for the Hubbard manifold. The derivative of the inverse square root
of the orbital overlap matrix is obtained as a closed-form solution of the
associated Lyapunov (Sylvester) equation. The expression for the resulting
contribution to the forces is presented in the framework of ultrasoft
pseudopotentials and the projector-augmented-wave method, and using a plane
wave basis set. We have benchmarked the present implementation with respect to
finite differences of total energies for the case of NiO, finding excellent
agreement. Owing to the accuracy of Hubbard-corrected density-functional theory
calculations - provided the Hubbard parameters are computed for the manifold
under consideration - the present work paves the way for systematic studies of
solid-state and molecular transition-metal and rare-earth compounds.Comment: 16 pages, 1 figur