Hubbard-corrected Density Functional Theory Research Articles

DFT + U Study of Plutonium Hydrides with Occupation Matrix Control.

The magnetic order of plutonium hydrides (PuHx) has long been a subject of controversy, both experimentally and theoretically. The magnetic, structural, electronic, and thermodynamic properties of PuHx are investigated using Hubbard-corrected density functional theory (DFT + U), with U derived from linear response calculations. To address the issue of electronic metastable states within DFT + U, we employ an occupation matrix control method as well as allowing 5f orbitals to break the structural symmetry. This advanced computational approach establishes that the ground-state magnetic order for PuH2 is antiferromagnetic and that for PuH3 is ferromagnetic, consistent with Faraday and nuclear magnetic resonance experiments. Using the conventional hydrogen-vacancy model for PuHx, the hydrogen-content-induced magnetic transition and anomalous variation of the magnetic moment are successfully reproduced. In particular, the calculated transition point of x matches the experimental findings. Furthermore, the electronic configuration of the Pu atom in PuHx is determined to be 5f5, which is consistent with observations from X-ray photoemission spectroscopy. Our calculated values for enthalpy of formation, heat capacity, and entropy are in good agreement with experimental data, thereby validating the robustness of our theoretical framework.

Influence of non-metals doping on the structural, electronic, optical, and photocatalytic properties of rutile TiO2 based on density functional theory computations

This study investigates the influence of non-metal doping (C, F, N, and S) on the structural, electronic, and optical properties of rutile TiO2 using Hubbard-corrected density functional theory (DFT) within the Quantum ESPRESSO code. Rutile TiO2 is a promising material for environmental remediation and renewable energy applications. However, it has a large bandgap of 3.03 eV, which confines its use to UV light. By substituting oxygen atoms with a single dopant atom, we aim to shift the absorption edge toward the visible spectrum. Our findings reveal that doping with C, N, and S results in a redshift of the bandgap, while F doping does not exhibit this effect. The imaginary part of the dielectric function indicates shifted absorption edges for C, N, and S-doped TiO2, making them suitable for photocatalytic applications. Additionally, an increased refractive index after doping suggests the presence of excess charge carriers that affect light propagation. This work provides valuable insights for experimentalists exploring non-metal doping influences on rutile TiO2 for photocatalysis.

Systematic Improvement of Quantum Monte Carlo Calculations in Transition Metal Oxides: sCI-Driven Wavefunction Optimization for Reliable Band Gap Prediction.

Accurate determination of the electronic properties of correlated oxides remains a significant challenge for computational theory. Traditional Hubbard-corrected density functional theory (DFT+U) frequently encounters limitations in precisely capturing electron correlation, particularly in predicting band gaps. We introduce a systematic methodology to enhance the accuracy of diffusion Monte Carlo (DMC) simulations for both ground and excited states, focusing on LiCoO2 as a case study. By employing a selected configuration interaction (sCI) approach, we demonstrate the capability to optimize wavefunctions beyond the constraints of single-reference DFT+U trial wavefunctions. We show that the sCI framework enables accurate prediction of band gaps in LiCoO2, closely aligning with experimental values and substantially improving traditional computational methods. The study uncovers a nuanced mixed state of t2g and eg orbitals at the band edges that is not captured by conventional single-reference methods, further elucidating the limitations of PBE+U in describing d-d excitations. Our findings advocate for the adoption of beyond-DFT methodologies, such as sCI, to capture the essential physics of excited-state wavefunctions in strongly correlated materials. The improved accuracy in band gap predictions and the ability to generate more reliable trial wavefunctions for DMC calculations underscore the potential of this approach for broader applications in the study of correlated oxides. This work not only provides a pathway for more accurate simulations of electronic structures in complex materials but also suggests a framework for future investigations of the excited states of other challenging systems.

Ternary Oxides of s - and p -Block Metals for Photocatalytic Solar-to-Hydrogen Conversion

Oxides containing metals or metalloids from the p block of the periodic table (e.g., In, Sn, Sb, Pb, and Bi) are of technological interest as transparent conductors and light absorbers for solar-energy conversion due to the tunability of their electronic conductivity and optical absorption. Comparatively, these oxides have found limited applications in hydrogen photoelectrolysis, primarily due to their high electronegativity, which impedes electron transfer for reducing protons into hydrogen. We have shown recently that inserting s-block cations into p-block metal oxides is effective at lowering electronegativities while affording further control of band gaps. Here, we explain the origins of this dual tunability by demonstrating the mediator role of s-block cations in modulating orbital hybridization while not contributing to frontier electronic states. From this result, we carry out a comprehensive computational study of 109 ternary oxides of s- and p-block metal elements as candidate photocatalysts for solar hydrogen generation. We down-select the most desirable materials using band gaps and band edges obtained from Hubbard-corrected density-functional theory, with Hubbard parameters computed entirely from first principles, evaluate the stability of these oxides in aqueous conditions, and characterize experimentally four of the remaining materials, synthesized with high phase uniformity, to validate and further develop the computational models. We thus propose nine oxide semiconductors, including CsIn3O5, Sr2In2O5, and KSbO2, which, to the extent of our literature review, have not been previously considered as water-splitting photocatalysts. Published by the American Physical Society 2024

Quantifying Local Atomic Distortions in UO2 Grain Boundaries: Correlation with Energetic and Electronic Properties.

UO2, as a key material in the nuclear industry, is composed of grains or crystallites in real applications. Their interfaces, known as grain boundaries (GBs), significantly impact thermal conductivity, corrosion resistance, and mechanical response. Here, utilizing Hubbard-corrected density functional theory, we systematically examine the local cluster structures, energetic stabilities, and electronic properties of five typical tilt UO2 GBs ranging from Σ 3 to Σ 11. We categorize all possible distorted U- and O-centered clusters at these GBs and identify their cluster morphologies and radial and angular distortions. Our results highlight the abundance of new U-O bonds stretching to "medium-range", a feature often overlooked in conventional coordination analysis. To quantitatively describe these distorted clusters, we use smooth overlap of atomic positions (SOAP) to represent the structural and chemical local environments, which takes into account both radial (2-body) and angular (3-body) distortions. We define a dissimilarity index by computing the inner product of SOAP descriptors between the distorted and the perfect motif in ideal UO2. Our findings show that the medium-range SOAP dissimilarity correlates well with the GB excess energy, outperforming metrics such as dangling bonds or bonding strain. Furthermore, it is found that the band gaps in sufficiently high-energy GBs are shortened, with excess states primarily contributed by the distorted U clusters. Our results present a comprehensive gallery of the local distorted clusters introduced by typical UO2 GBs and have implications on the structure-property relations of GBs and other interfaces.

Valence band electronic structure of the van der Waals antiferromagnet FePS3

Antiferromagnetic van der Waals materials have gained a lot of interest in recent years. They can be exfoliated down to the two-dimensional (2D) limit while potentially preserving intriguing properties of antiferromagnets, such as insensitivity to external magnetic fields and ultrafast spin dynamics in the THz range. The investigation of the electronic band structure of these materials is crucial to understand their behavior and thus to identify paths for future applications. Here, we investigate the valence band structure of one of the most studied 2D antiferromagnets –iron phosphorus trisulfide (FePS3)– using angle-resolved photoemission spectroscopy (ARPES) and compare our results with first-principles calculations based on Hubbard-corrected density functional theory (DFT+U). This allows us to identify the bands originating respectively from the Fe 3d, the S 3p, and the P 3p orbitals and to describe their dispersion throughout the whole Brillouin zone. Our results represent an important step towards an accurate theoretical description of the electronic properties of transition metal phosphorus trisulfides, which is a pre-requisite for understanding the behavior of antiferromagnetic materials at the 2D limit.

Electronic-Level Insight into the Adsorption and Surface Diffusion Kinetics of a Simplified Glyphosate Model on a Goethite Surface.

The diffusive processes that occur in minerals involve chemical and physical surface phenomena of great interest that allow for understanding the mobility of different anions of environmental importance. One of them is glyphosate, which is widely used as a pesticide. In this work, we performed Hubbard-corrected density functional theory (DFT + U) calculations to study the adsorption and surface diffusion of methylphosphonic acid (MPA), as a model of glyphosate, on the (010) plane of goethite (GOT), one of the most important Fe(III) minerals in soils and sediments. In particular, the MPA adsorption was studied at the GOT-water interface, finding a strong covalent character in the bond. We also corroborated the occurrence of double proton transfer (MPA to GOT and GOT to GOT). Finally, activation energy barriers were calculated to estimate the half-lives for molecular diffusion, showing that MPA moves almost 3000 times slower than water at the GOT surface.

Theoretical prediction of two-dimensional Cr3C2 monolayer and its derivatives as potential electrode of Li-ion batteries

The investigations on Cr3C2 and Cr3C2T2 (T = F, Cl, O, OH) monolayers as promising electrode materials for lithium-ion battery (LIBs) are systematically carried out by Hubbard-corrected density functional theory (DFT + U). It indicates that Cr3C2 monolayer and its derivatives are dynamically and thermally stable. At the same time, the pristine Cr3C2 monolayer exhibits metallic character and the conductivity can be maintained through chemical modification. The lithium diffusion barrier on Cr3C2 monolayer is only 0.024 eV, which can guarantee a fast rate during the charge/discharge process. The storage capacity of Cr3C2 monolayer adsorbing one layer of Li atoms is about 298 mAh/g. It is found that the Cr3C2T2 (T = Cl, O, OH) monolayer can keep at least one layer of Li atoms adsorbed without any structural deformation, and the theoretical capacities are 214, 253 and 250 mAh/g, respectively. The Li storage capacities can be increased exponentially with the more layers of Li atoms adsorbing on the two-dimensional (2D) materials. Thus, Cr3C2 and Cr3C2T2 monolayers are potential electrode materials for LIBs.

A deep investigation of NiO and MnO through the first principle calculations and Monte Carlo simulations

In this study, we use Hubbard-corrected density functional theory (DFT+U) to derive spin model Hamiltonians consisting of Heisenberg exchange interactions up to the fourth nearest neighbors and bi-quadratic interactions. We map the DFT+U results of several magnetic configurations to the Heisenberg spin model Hamiltonian to estimate Heisenberg exchanges. We demonstrate that the number of magnetic configurations should be at least twice the number of exchange parameters to estimate exchange parameters correctly. To calculate biquadratic interaction, we propose specific non-collinear magnetic configurations that do not change the energy of the Heisenberg spin model. We use classical Monte Carlo (MC) simulations to evaluate DFT+U results. We obtain the temperature dependence of magnetic susceptibility and specific heat to determine the Curie–Weiss and Néel temperatures. The MC simulations reveal that although the biquadratic interaction can not change the Néel temperature, it modifies the order parameter. We indicate that for a fair comparison between classical MC simulations and experiments, we need to consider the quantum effect by applying correction in classical MC simulations.

Modeling Terahertz Properties of Vanadium Dioxide by Ab Initio Computational Scheme and its Experimental Verification

A complete simulation scheme is established to model the terahertz (THz) properties of vanadium dioxide (${\mathrm{VO}}_{2}$) from ab initio to computational electromagnetic method. For the monoclinic insulating phase of ${\mathrm{VO}}_{2}$, dielectric properties are computed by Hubbard-corrected density functional perturbation theory. Meanwhile, the electron-phonon interactions are studied to model the electronic transport properties of the tetragonal metallic phase of ${\mathrm{VO}}_{2}$. In order to build a bridge from the microscopic effects of first-principles calculations to the macroscopic THz properties, the Drude model and effective medium theory are applied to model the dispersion properties of the material. To validate the proposed model, a ${\mathrm{VO}}_{2}$ film grown on a sapphire substrate is fabricated and measured by a terahertz time-domain spectroscopy system. The measurement results have excellent agreements with the proposed simulations. Further, a ${\mathrm{VO}}_{2}$-based THz metamaterial modulator is simulated by using the proposed model, in great agreement with the reported measurement result. Our ab initio computational scheme improves the simulation accuracy of design, which provides a solid THz modeling approach to the ${\mathrm{VO}}_{2}$ and other insulator-metal transition metal oxides.

Ortho-atomic projector assisted DFT+U study of room temperature Ferro- and antiferromagnetic Mn-doped TiO2 diluted magnetic semiconductor

Using ortho-atomic Hubbard-corrected density functional theory, we present magnetic properties, ferromagnetic transition temperature T c , Neel temperature T N , electronic structure, structural formation energy, and crystal structure of anatase (Ti16−x Mn x O32 for x = 1, 2, 3, 4 and 6). According to the calculated formation energy, an oxygen-rich condition is more structurally stable than a Ti-rich situation. The geometric and lattice parameter optimization result indicates that Mn2+ exists in the system. In addition, when Mn is substituted in the system, some of the bonds that connect oxygen with the Mn ion are broken, and the bond lengths show variation; this results in defects, which cause the formation of oxygen vacancies that enhance both ferromagnetic and antiferromagnetic properties of the system. According to the structure of electronic bands and PDOS analysis, 2p and 3d hybridization lead to a decrease in the edge of the CB and an increase in the edge of the VB, narrowing the bandgap from 3.23 eV to 2.21 eV(for 4.17% of Mn), 2.13 eV (for 8.33% of Mn) and 1.69 eV (for 12.5% of Mn). Results also showed that both ferromagnetic and antiferromagnetic properties are energetically favored. Furthermore, the material exhibits antiferromagnetic with predicted T N to be 804.13 K for a small concentration of Mn dopant (for 4.17% of Mn), which is higher than room temperature. In contrast, the calculated ferromagnetic temperature (T c ) was increased from 255.42 K (for 8.33% of Mn) to 385.63 K (for 12.5% of Mn), most likely as a cause of bound magnetic polarons (BMPs) production. Result also reveals oxygen vacancy for the relaxed pristine (Ti16O31) a non-zero magnetic moment and a total magnetization of 2.00 μ B which is observed as a result of an ortho-atomic projector. The present findings suggest that TiO2 could be tailored through doping and oxygen vacancy for versatile spintronic applications, inconsistent with previous experimental results.

Influence of Th, Zr, and Ti Dopants on Solution Property of Xe in Uranium Dioxide with Defects: A DFT + U Study

To ensure the safety and efficient operation of nuclear reactors, it is imperative to understand the effects of various dopants (Ti, Th, and Zr) on the solubility of the fission product Xe in UO2. In this study, Hubbard corrected density functional theory (DFT + U) and occupation matrix control were used to investigate the bulk and defect properties of UO2. The results show that the UO2-Ti system is more favorable for Xe dissolution in vacancies, whereas the UO2-Th system has little effect on the dissolution of Xe atoms. Th, Zr, and Ti inhibit the aggregation of Xe clusters, and Ti is the least favorable for the nucleation and growth of Xe clusters.

Theoretical Study on Photocatalytic CO2 Reduction to CO and CH4 over M(II)2M(III/IV)-Layered Double Hydroxides

Photocatalytic CO2 reduction (CO2PR) is of great significance for high-density solar energy storage and carbon balance, and layered double hydroxides (LDHs) as CO2PR catalysts have attracted much attention. In this work, the electronic properties such as the band structure, density of states and band edge placement of 10 MII2MIII/IV-NO3-LDHs (MII = Mg2+, Co2+, Ni2+, Zn2+; MIII = Al3+, In3+, Cr3+, Fe3+; MIV = Ti4+), thermodynamic mechanism, and thermodynamically favorable products of CO2PR, especially CO and CH4, over these LDHs are investigated by using Hubbard-corrected density functional theory. The calculation results indicate that all LDHs except the Mg2Al- and Mg2In-LDHs are responsive to visible light, and all the LDHs have enough driving force to undergo CO2PR to products except the Ni2Al- and Ni2Fe-LDHs. For Mg- and Zn-containing LDHs, the COOH pathway is favorable, while the Ni2Ti- and Co2Al-LDHs prefer the HCOO pathway. According to the minimum-free-energy reaction pathway, for Co2Fe-LDH, CO2PR prefers to generate HCOOH, and the potential-determining step is HCOO* → HCOOH*. For the Mg2Al-, Mg2In-, Co2Al-, Ni2Ti-, Zn2Al-, Zn2Cr-, and Zn2Ti-NO3-LDHs, CH4 is the favorable final product via HCOOH, HCHO, or CH3OH intermediates. The formation of CO mainly goes through the COOH pathway. The relationship between the effective driving force of CO2PR to CH4 or CO and the adsorption energy of the reactant CO2 or the valence band maximum (VBM) of LDHs reveals that the Mg2In-LDH and Mg2Al-LDH, which have a moderate adsorption and VBM favor CO2PR to CH4 and CO the most, respectively. On the Co2Fe-LDH (110), CO2 prefers to form CH4 than that of the Co2Fe-LDH (003), which indicates that the crystal faces also affect favorable products of CO2PR. This work provides useful information of the photocatalytic performance of LDH materials for CO2PR and is helpful for the design of highly efficient CO2PR catalysts.

AbInitio Electron-Phonon Interactions in Correlated Electron Systems.

Electron-phonon (e-ph) interactions are pervasive in condensed matter, governing phenomena such as transport, superconductivity, charge-density waves, polarons, and metal-insulator transitions. First-principles approaches enable accurate calculations of e-ph interactions in a wide range of solids. However, they remain an open challenge in correlated electron systems (CES), where density functional theory often fails to describe the ground state. Therefore reliable e-ph calculations remain out of reach for many transition metal oxides, high-temperature superconductors, Mott insulators, planetary materials, and multiferroics. Here we show first-principles calculations of e-ph interactions in CES, using the framework of Hubbard-corrected density functional theory (DFT+U) and its linear response extension (DFPT+U), which can describe the electronic structure and lattice dynamics of many CES. We showcase the accuracy of this approach for a prototypical Mott system, CoO, carrying out a detailed investigation of its e-ph interactions and electron spectral functions. While standard DFPT gives unphysically divergent and short-ranged e-ph interactions, DFPT+U is shown to remove the divergences and properly account for the long-range Fröhlich interaction, allowing us to model polaron effects in a Mott insulator. Our work establishes a broadly applicable and affordable approach for quantitative studies of e-ph interactions in CES, a novel theoretical tool to interpret experiments in this broad class of materials.

Atomistic investigation of uranium oxycarbide (UCO) phase stability at finite temperatures: DFT+U and AIMD+U approaches

Materials properties of uranium oxycarbide (UCO), a fuel candidate for high temperature gas reactors (HTGRs), are investigated using the Hubbard corrected density functional theory (DFT+ U ) and ab initio molecular dynamics (AIMD+ U ). To accurately describe UCO systems within the supercell approach, we have evaluated the structural disorder of the ion sublattice within the special quasirandom structures (SQS) approach. The ground-state of UC x O 2-x and UC 1-x O x systems was found by examination of the electronic configuration, the occupation matrix and the Hubbard U parameter. We obtained the electronic properties, lattice structure, formation enthalpy and free energy of mixing of the stoichiometric UC x O 2-x and UC 1-x O x phases. Subsequently, we have determined the preferential phase of UCO (UC x O 2-x or UC 1-x O x ) for low carbon and oxygen concentrations, respectively. In particular, UC x O 2-x was found stable for carbon content lower than 50%, with carbon atoms in the first coordination shell out of the perfect fluorite positions, driven by the formation of C 2 groups as in α -UC 2 . Moreover, the analysis of the radial distribution functions shows that structures with C 2 groups remain stable in the range of temperatures from 300 to ∼ 2500 K.

Magnetic Energy Landscape of Dimolybdenum Tetraacetate on a Bulk Insulator Surface

The magnetic states and the magnetic anisotropy barrier of a transition metal molecular complex, dimolybdenum tetraacetate, are investigated via density functional theory (DFT). Calculations are performed in the gas phase and on a calcite (10.4) bulk insulating surface, using the Generalized-Gradient Approximation (GGA)-PBE and the Hubbard-corrected DFT + U and DFT + U + V functionals. The molecular complex (denoted MoMo) contains two central metallic molybdenum atoms, embedded in a square cage of acetate groups. Recently, MoMo was observed to form locally regular networks of immobile molecules on calcite (10.4), at room conditions. As this is the first example of a metal-coordinated molecule strongly anchored to an insulator surface at room temperature, we explore here its magnetic properties with the aim to understand whether the system could be assigned features of a single molecule magnet (SMM) and could represent the basis to realize stable magnetic networks on insulators. After an introductory review on SMMs, we show that, while the uncorrected GGA-PBE functional stabilizes MoMo in a nonmagnetic state, the DFT + U and DFT + U + V approaches stabilize an antiferromagnetic ground state and several meta-stable ferromagnetic and ferrimagnetic states. Importantly, the energy landscape of magnetic states remains almost unaltered on the insulating surface. Finally, via a noncollinear magnetic formalism and a newly introduced algorithm, we calculate the magnetic anisotropy barrier, whose value indicates the stability of the molecule’s magnetic moment.

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 $U$ to also compute the inter-site Hubbard $V$. 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 Li$_x$MnPO$_4$ (x=0,1) and the subtle energetics of Li intercalation.

Metallic and anti-metallic properties of hydrogen adsorbed AnO2 (An = Th, U, and Pu) surfaces

The effect of atomic hydrogen adsorption on AnO2 (An = Th, U, and Pu) surfaces is studied in the framework of density functional theory and Hubbard-corrected density functional theory. Several adsorption coverages (1/3, 1/2, 2/3, and 1 monolayer) are considered. For the band insulator ThO2, surface metallicity induced by hydrogen adsorption is observed due to the electron donation of the hydrogen to the surface. But this effect is found to be strongly suppressed by electronic correlation for the Mott insulators UO2 and PuO2 because the electrons from the adsorbed hydrogen atoms occupy the localized 5f orbitals of the surface U/Pu atoms.

Fast 3D-lithium-ion diffusion and high electronic conductivity of Li2MnSiO4 surfaces for rechargeable lithium-ion batteries.

High theoretical capacity, high thermal stability, the low cost of production, abundance, and environmental friendliness are among the potential attractiveness of Li2MnSiO4 as a positive electrode (cathode) material for rechargeable lithium-ion batteries. However, the experimental results indicated poor electrochemical performance in its bulk phase due to high intrinsic charge transfer resistance and capacity fading during cycling, which limit its large-scale commercial applications. Herein, we explore the surface stability and various lithium-ion diffusion pathways of Li2MnSiO4 surfaces using the density functional theory (DFT) framework. Results revealed that the stability of selected surfaces is in the following order: (210) > (001) > (010) > (100). Moreover, the Wulff-constructed equilibrium shape revealed that the Li2MnSiO4 (001) surface is the most predominant facet, and thus, preferentially exposed to electrochemical activities. The Hubbard-corrected DFT (DFT + U, with U = 3 eV) results indicated that the bulk insulator with a wide band gap (Eg = 3.42 eV) changed into narrow electronic (Eg = 0.6 eV) when it comes to the Li2MnSiO4 (001) surface. Moreover, the nudged elastic band analysis shows that surface diffusion along the (001) channel was found to be unlimited and fast in all three dimensions with more than 12-order-of-magnitude enhancements compared with the bulk system. These findings suggest that the capacity limitation and poor electrochemical performance that arise from limited electronic and ionic conductivity in the bulk system could be remarkably improved on the surfaces of the Li2MnSiO4 cathode material for rechargeable lithium-ion batteries.

Formation and stability of small polarons at the lithium-terminated Li4Ti5O12 (LTO) (111) surface.

Zero strain insertion, high cycling stability, and a stable charge/discharge plateau are promising properties rendering Lithium Titanium Oxide (LTO) a possible candidate for an anode material in solid state Li ion batteries. However, the use of pristine LTO in batteries is rather limited due to its electronically insulating nature. In contrast, reduced LTO shows an electronic conductivity several orders of magnitude higher. Studying bulk reduced LTO, we could show recently that the formation of polaronic states can play a major role in explaining this improved conductivity. In this work, we extend our study toward the lithium-terminated LTO (111) surface. We investigate the formation of polarons by applying Hubbard-corrected density functional theory. Analyzing their relative stabilities reveals that positions with Li ions close by have the highest stability among the different localization patterns.