On-site Coulomb Correction Research Articles

Doubly Screened Coulomb Correction Approach for Strongly Correlated Systems.

Strongly correlated systems containing d/f electrons present a challenge to conventional density functional theory such as the local density approximation or generalized gradient approximation. We developed a doubly screened Coulomb correction (DSCC) approach to perform on-site Coulomb interaction correction for strongly correlated materials. The on-site Coulomb interaction between localized d/f electrons is self-consistently determined from a model dielectric function that includes both the static dielectric and Thomas-Fermi screening. We applied DSCC to simulate the electronic and magnetic properties of typical 3d, 4f, and 5f strongly correlated systems. The accuracy of DSCC is comparable to that of hybrid functionals but an order of magnitude faster. In addition, DSCC can reflect the difference in the Coulomb interaction between metallic and insulating situations, similar to the popular but computationally expensive constrained random phase approximation approach. This feature suggests that DSCC is also a promising method for simulating Coulomb interaction parameters.

Fabrication of supported Pt/CeO2 nanocatalysts doped with different elements for CO oxidation: theoretical and experimental studies.

Supported Pt/CeO2 catalysts have been widely used in carbon monoxide (CO) oxidation; however, the high oxygen vacancy formation energy (Evac) in the process leads to the poor performance of these catalysts. Herein, we explored different element (Pr, Cu, or N) doped CeO2 supports using Ce-based metal-organic frameworks (MOFs) as precursors via calcination treatment. The obtained CeO2 supports were used to load Pt nanoparticles. These catalysts were systematically characterized by various techniques, and they showed superior catalytic activity for CO oxidation compared to undoped catalysts which could be attributed to the formation of Ce3+, and high amounts of Oads/(Oads + Olat) and Ptδ+/Pttotal. Moreover, density functional theory calculations with on-site Coulomb interaction correction (DFT+U) were performed to provide atomic-scale insights into the reaction process by the Mars-van Krevelen (M-vK) mechanism, which revealed that the element-doped catalysts could simultaneously reduce the adsorption energies of CO and lower reaction energy barriers in the *OOCO associative pathway.

Impact of Anionic Substitution in Yb14MgSb11-x As x Compounds on the Electronic and Thermoelectric Properties.

The effects of anionic site substitution on the electronic transport properties of Yb14MgSb11-x As x compounds were investigated using density functional theory (DFT) with on-site Coulomb interaction correction (PBE+U). By replacing the Sb atoms at the four symmetry sites in Yb14MgSb11 with As, we found that the electronic and thermoelectric properties of the compound can be altered substantially. For most of the cases, the thermoelectric properties improve compared to the base compound Yb14MgSb11. Substitution at the tetrahedral site (Sb2) in particular yields the highest improvement in the thermoelectric properties. Detailed insight into the electronic and structural changes caused by the selective site substitutions is also discussed.

The structural, elastic, electronic, magnetic and optical properties of SrNiO3 perovskite: A DFT and DFT+U study

Transition-metal based half-metallic oxide perovskites are the phenomenal prospect of spintronics for their expectancy of success in information technology. Generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional and also adding on-site Coulomb interaction correction (GGA-PBE+U) is implemented within density functional theory (DFT) to analyze the structural, elastic, electronic, magnetic, and optical properties of the oxide perovskites SrNiO3. The structural properties such as lattice parameter, cell volume, total energy, bulk modulus, and pressure derivative are computed at zero pressure. The elastic constants corroborate the material’s mechanical stability and stiffness. The Cauchy pressure, bulk/shear ratio, and Poisson’s ratio values indicate that this material is ductile. The electronic properties, including band structures, total and partial density of states, especially hybridization among Ni 3d and O 2p orbitals, result in half-metallic ferromagnetism. The magnetic moment of the Ni atom keeps rising gradually as the U value increases. Electronic structures are used to analyze and explain the real and imaginary parts of the dielectric function, absorption coefficient, energy loss function, reflectivity, refractive index, and extinction coefficient. Our findings suggest that this compound could be an excellent fit for spintronics applications.

Crystal Chemistry and Electronic Properties of the Al-Rich Compounds, Al2Cu, ω-Al7Cu2Fe and θ-Al13Fe4 with Cu Solution

In this work, we investigate Cu solution in θ-Al13Fe4 and related Al-rich ω-Al7Cu2Fe and Al2Cu phases in the Al-Cu-Fe system using the first-principles density functional theory (DFT) with on-site Coulomb interaction correction. The results show preference of Cu at Al7, forming a ternary θ-Al76Cu2Fe24 at ambient conditions, and both Al7 and Al9 sites (in Grin’s note), forming θ-(Al76−xCu2+x)Fe24 at a high temperature. The relative stability of the Al-rich compounds and their crystal and electronic properties are investigated. We show the importance of the Hubbard U correction to the standard DFT functionals for Cu-containing metallic materials. This study helps characterize the intermetallic compounds in Cu-containing Al alloys, and helps further control Fe-containing intermetallic compounds in the solidification of Al-based alloys.

Interfacial coupling effects on adsorptive and photocatalytic performances for photoresponsive graphene-wrapped SrTiO3@Ag under UV–visible light: experimental and DFT approach

Understanding the graphene/semiconductor/metal interactions is crucial to design innovative photocatalytic materials with efficient photocatalytic activity for environmental cleanup applications. SrTiO3 on reduced graphene oxide (rGO) with various graphene contents was successfully synthesized in this study utilizing a simple hydrothermal method, followed by decorating the surface with Ag particles by using the photodeposition process. Under UV-visible light irradiation, the resulting composites were tested for their improved photocatalytic activity to decompose methylene blue (MB). The prepared photocatalysts were characterized by XRD, SEM, EDX, DLS, FT-IR, Raman spectroscopy, and DRS. First-principle density functional theory calculations (DFT) were also carried out by using the generalized gradient approximation (GGA) and PBE functional with the addition of on-site Coulomb correction (GGA + U). The obtained SrTiO3/rGO@Ag composites showed great improvement in the photocatalytic performances over pristine SrTiO3. For the degradation reaction of MB, SrTiO3/rGO20%@Ag4% composites yielded the best photocatalytic activity with efficacy reach 94%, which was also shown that it could be recycled up to four times with nearly unchanged photocatalytic activity.

Catalytic formation of oxalic acid on the partially oxidised greigite Fe3S4(001) surface.

Greigite (Fe3S4), with its ferredoxin-like 4Fe-4S redox centres, is a naturally occurring mineral capable of acting as a catalyst in the conversion of carbon dioxide (CO2) into low molecular-weight organic acids (LMWOAs), which are of paramount significance in several soil and plant processes as well as in the chemical industry. In this paper, we report the reaction between CO2 and water (H2O) to form oxalic acid (H2C2O4) on the partially oxidised greigite Fe3S4(001) surface by means of spin-polarised density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+U-D2). We have calculated the bulk phase of Fe3S4 and the two reconstructed Tasker type 3 terminations of its (001) surface, whose properties are in good agreement with available experimental data. We have obtained the relevant phase diagram, showing that the Fe3S4(001) surface becomes 62.5% partially oxidised, by replacing S by O atoms, in the presence of water at the typical conditions of calcination [Mitchell et al. Faraday Discuss. 2021, 230, 30-51]. The adsorption and co-adsorption of the reactants on the partially oxidised Fe3S4(001) surface are exothermic processes. We have considered three mechanistic pathways to explain the formation of H2C2O4, showing that the coupling of the C-C bond and second protonation are the elementary steps with the largest energy penalty. Our calculations suggest that the partially oxidised Fe3S4(001) surface is a mineral phase that can catalyse the formation of H2C2O4 under favourable conditions, which has important implications for natural ecosystems and is a process that can be harnessed for the industrial manufacture of this organic acid.

First-principles investigations on the electronic structures, polycrystalline elastic properties, ideal strengths and elastic anisotropy of U3Si2

First-principles calculations based on density functional theory with on-site Coulomb correction were used to examine the electronic structures, polycrystalline mechanical properties, ideal tensile and shear strengths and elastic anisotropy of U3Si2. The present lattice parameters and elastic stiffness constants of single crystal are in good agreement with the experimental data and other theoretical results. Density functional perturbation theory method was used to obtain the dynamical properties of U3Si2, and the well-known Born criteria was investigated to prove the structural stability of the ideal strength calculation model. Also, Voigt–Reuss–Hill model was used to evaluate the polycrystalline properties such as Young’s modulus, the shear modulus, the bulk modulus and so on. The semiempirical formulas for the theoretical estimation of the Debye temperature were adopted. The first-principles computational tensile test (FPCTT) was used to explore the ideal tensile strengths along typical crystal orientations and the ideal shear strengths in several slip systems of P4/mbm U3Si2. Additionally, the elastic anisotropy of P4/mbm U3Si2 was characterized by several anisotropic factors and directional moduli. Within the elastic strain range, the Poisson’s ratios under different single loads were obtained. The least-squares method was used for polynomial fitting of the data obtained by FPCTT method, and the results were satisfactory. These calculated results help to understand the nature of U3Si2 better and provide more useful information to design and develop U3Si2 for a new fuel-cladding solution. The relation curves between stress and strain of U3Si2 were obtained and the ideal tensile and shear strengths of U3Si2 were predicted.

Function-oriented design of robust metal cocatalyst for photocatalytic hydrogen evolution on metal/titania composites

While the precise design of catalysts is one of ultimate goals in catalysis, practical strategies often fall short, especially for complicated photocatalytic processes. Here, taking the hydrogen evolution reaction (HER) as an example, we introduce a theoretical approach for designing robust metal cocatalysts supported on TiO2 using density functional theory calculations adopting on-site Coulomb correction and/or hybrid functionals. The approach starts with clarifying the individual function of each metal layer of metal/TiO2 composites in photocatalytic HER, covering both the electron transfer and surface catalysis aspects, followed by conducting a function-oriented optimization via exploring competent candidates. With this approach, we successfully determine and verify bimetallic Pt/Rh/TiO2 and Pt/Cu/TiO2 catalysts to be robust substitutes for conventional Pt/TiO2. The right metal type as well as the proper stacking sequence are demonstrated to be key to boosting performance. Moreover, we tentatively identify the tunneling barrier height as an effective descriptor for the important electron transfer process in photocatalysis on metal/oxide catalysts. We believe that this study pushes forward the frontier of photocatalyst design towards higher water splitting efficiency.

DFT+U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

ABSTRACT Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions. Keywords: Materials chemistry, cobalt oxides, cobalt metal, spinel, density functional theory, on-site Coulomb correction.

Oxygen-Mediated Superexchange Interactions and Their Impact on the Structural Stability, Magnetic Order, and Magnetocrystalline Anisotropy of One-Dimensional Co-Oxide Chains on Rh(553) Step-Surfaces

First-principles calculations in the framework of the generalized gradient approximation together with U on-site Coulomb corrections in the GGA + U approach to density functional theory (DFT) are p...

Structural, electronic and optical properties of S-doped, Sc-doped and Sc–S co-doped anatase TiO2: a DFT + U calculation

Defect formation energies, electronic structures and optical properties of non-metal S, lanthanide metal Sc-doped and Sc–S co-doped anatase TiO2 were systematically investigated via a GGA + U method based on density functional theory (DFT). The on-site Coulomb corrections were applied to both 3d orbitals of Ti atom (U Ti _3d ) and 2p orbitals of O atom (U O _2p ). The results show all doped TiO2 systems exhibit a certain degree of red-shift due to the presence of impurity levels in the bandgap; in particular, the S-doped and Sc–S co-doped TiO2 exhibit better light absorption properties in the visible light range than either pure or Sc-doped TiO2 . In addition, the positive synergistic effect between the Sc and S atoms not only expands the absorption edge, but also increases the light absorption coefficient in the visible region. As a result, the Sc–S co-doped anatase TiO2 shows the best optical absorption spectrum in all doping systems.

DFT calculations and in situ DRIFTS study of CO oxidation on CeO2/Co3O4 catalyst

The surface structures, CO adsorption, and oxidation-reaction properties of CeO2/Co3O4(110) have been investigated by using density functional theory including on-site Coulomb corrections (DFT+U) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Results indicate that CO can be chemisorbed on the Co sites of the terrace and interface of CeO2/Co3O4(110). At the interface, adsorbed and gas-phase CO reacting with lattice oxygen are investigated, respectively. It has been found that the lattice oxygen at the interface is more likely to be attacked by gas-phase CO directly to form adsorbed CO2. For desorption of the CO2, very high energy barriers are required. Furthermore, the infrared spectra reveal that the intermediate species at interface still exist even at 300 °C.

Structures and reactivities of the CeO2/Pt(111) reverse catalyst: A DFT+U study

AbstractFor heterogeneous catalysts, the build-up of interface contacts can influence markedly their activities. Being different from the conventional supported metal/oxide catalysts, the reverse type of oxide/metal structures, e.g. the ceria/Pt composite, have emerged as novel catalytic materials in many fields. However, it remains challenging to determine the optimal interface structure and/or the metal-oxide synergistic effect that can boost catalytic activities. In this work, we conducted density functional theory calculations with on-site Coulomb interaction correction to determine the optimal structures and investigate the physical as well as catalytic properties of various CeO2/Pt(111) composites containing CeO2(111) monolayer, bilayer, and trilayer at Pt(111). We found that the interaction strength between CeO2(111) and Pt(111) substrate first reduces as the ceria slab grows from monolayer to bilayer, and then largely gets converged when the trilayer occurs. Such trend was well rationalized by analyzing the number and distances of O–Pt bonds at the interface. Calculated Bader charges uncovered the significant charge redistribution occurring around the interface, whereas the net electron transfer across the interface is non-significant and decreases as ceria thickness increases. Moreover, comparative calculations on oxygen vacancy formation energies clarified that oxygen removal can be promoted on the CeO2/Pt(111) composites, especially at the interface. We finally employed CO oxidation as a model reaction to probe the surface reactivity, and determined an intrinsic activity order of monolayer CeO2(111) > monolayer CeO2(111)/Pt(111) > regular CeO2(111). More importantly, we emphasized the significant role of the moderate ceria-Pt interaction at the interface that endows the CeO2/Pt reverse catalyst both good thermostability and high catalytic activity. The monolayer CeO2(111)/Pt(111) composite was theoretically predicted highly efficient for catalyzing CO oxidation.

Structural and Electronic Properties of Bulk ZnX (X = O, S, Se, Te), ZnF2, and ZnO/ZnF2: A DFT Investigation within PBE, PBE + U, and Hybrid HSE Functionals.

Here, we have studied the crystalline structure of bulk ZnX (X = O, S, Se, Te) and ZnF2 systems as a first step to understand the structures like ZnX and Zn-based systems like ZnO/ZnF2 interfaces, which are of utmost importance for possible technological applications. In addition, an adequate methodological description based on density functional theory (DFT) calculations is necessary. It is well known that plain DFT calculations based on local or semilocal exchange-correlation functionals fail to describe the correct band gap energy for these systems, whereas nonlocal approaches, such as hybrid-based functionals, can compensate the underestimation of band gap. To contribute to the assessment, DFT studies were performed within semilocal Perdew-Burke-Ernzerhof (PBE) and two nonlocal functionals, hybrid Heyd-Scuseria-Ernzerhof (HSE) and PBE + U functionals. Our results confirm that PBE underestimates the energy band gap values, from 33.0 to 42.8% for ZnX compounds compared to the experimental values. Applying the hybrid HSE functional, we obtained a band gap dependency in relation to the range of separation of the nonlocal exact exchange, in general decreasing the band gap error and improving the lattice constant description. In addition, using the PBE + U approach, we have investigated the localization of the Zn d-states and its effect on the band gap in ZnX and ZnF2. We found an increase in the band gap with increasing Hubbard parameter, which introduces on-site Coulomb corrections for the Zn 3d states. In the same context, the relevance to include the Hubbard corrections for the O 2p states (and X p states) is highlighted. Thus, considering PBE + U, the error in ZnO band gap, for example, decreases to 5.1%, in relation to the experimental value. Finally, ZnO-12L/ZnF2-4L superlattices are found to exhibit conventional electronic properties, such as low fundamental band gap, smaller than either of the parent materials. Our first-principles calculations reveal that the unexpected band gap reduction is induced by the conducting layers that tend to penetrate the interface and decrease the band gap, leading to the transport of carriers through the interface to ZnF2, which, even with a high band gap for charge transfer, can be interesting for photovoltaic applications.

Ethylene carbonate adsorption on the major surfaces of lithium manganese oxide Li1-xMn2O4 spinel (0.000 < x < 0.375): a DFT+U-D3 study.

Understanding the surface reactivity of the commercial cathode material LiMn2O4 towards the electrolyte is important to improve the cycling performance of secondary lithium-ion batteries and to prevent manganese dissolution. In this work, we have employed spin-polarized density functional theory calculations with on-site Coulomb interactions and long-range dispersion corrections [DFT+U-D3-(BJ)] to investigate the adsorption of the electrolyte component ethylene carbonate (EC) onto the (001), (011) and (111) surfaces of the fully lithiated and partially delithiated Li1-xMn2O4 spinel (0.000 < x < 0.375). The surface interactions were investigated by evaluating the adsorption energies of the EC molecule and the surface free energies. Furthermore, we analyzed the impact of EC adsorption on the Wulff crystal morphologies, the molecular vibrational frequencies and the adsorbate/surface charge transfers. The adsorption energies indicate that the EC molecule strongly adsorbs on the (111) facet, which is attributed to a bidentate binding configuration. We found that EC adsorption enhances the stability of the (111) facet, as shown by the Wulff crystal morphologies. Although a negligible charge transfer was calculated between the spinel surfaces and the EC molecule, a large charge rearrangement takes place within the surfactant upon adsorption. The wavenumbers of the C[double bond, length as m-dash]O stretching mode for the interacting EC molecule are red-shifted with respect to the isolated adsorbate, suggesting that this bond becomes weaker. The surface free energies show that both the fully lithiated and partially delithiated forms of the LiMn2O4 surfaces are stabilized by the EC molecule.

Electronic structures, elastic, magnetic properties and half-metallicity in PtONa3 anti-perovskite

The structural, elastic, electronic and magnetic properties of the cubic [Formula: see text] anti-perovskite are investigated by means of the full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). We have used three approximations: the generalized gradient (GGA), the GGA+[Formula: see text][Formula: see text], where [Formula: see text] is on-site Coulomb interaction correction, and the modified Becke–Johnson (mBJ-GGA). The elastic constants [Formula: see text] show that our compounds are ductile and anisotropic. The results obtained for the spin-polarized band structure and the density of states show a half-metallic behavior for the compounds using the GGA, GGA+[Formula: see text][Formula: see text] and mBJ-GGA approaches. These results make [Formula: see text] a promising candidate for spintronics applications.

First-principles study of the structural and electronic properties of bulk ZnS and small ZnnSn nanoclusters in the framework of the DFT+U method

We investigate the structural and electronic properties of bulk ZnS in zinc blende as well as in wurtzite phase, and ${\mathrm{Zn}}_{n}{\mathrm{S}}_{n}$ nanoclusters by using the Hubbard model $(\mathrm{DFT}+U)$. It provides an on-site Coulomb correction to mitigate some of the commonly known limitations of traditional DFT-GGA method such as the underestimation of band gap and inaccurate description of electronic band structure. Especially for the nanoclusters, the traditional DFT method cannot reproduce all properties accurately that are observed in the experiments. Within the framework of $\mathrm{DFT}+U$ method, our model is first able to predict various properties of bulk ZnS (zinc blende and wurtzite) as well as in nanoclusters with high accuracy. We empirically determined the Hubbard correction parameters ${U}_{d}$ and ${U}_{p}$ for $\mathrm{Zn}\text{\ensuremath{-}}3d$ and $\mathrm{S}\text{\ensuremath{-}}3p$ states, respectively, that could reproduce the measured values of band gaps, $d$-band positions, $p$-states bandwidths, lattice parameters, etc. with a reasonable agreement. It was found that our model can be compared very well with more accurate hybrid functionals at only a fraction of the computational cost. Further, the selected pair of ${U}_{d}$ and ${U}_{p}$ values are used to investigate the structural and electronic properties of ZnS nanoclusters and results agree well with the higher levels of theory.

Investigation of the on-site Coulomb correction and temperature dependence of the stability of U–Si phases using DFT+U

The development and qualification of silicide-based nuclear fuel would significantly benefit from an adequate understanding of the U–Si phase space, yet issues remain with respect to confidence in the phase diagram, especially in the USi–USi2 composition range. Experimental investigations have shown the existence of hexagonal and tetragonal phases in the USix 1.5 < x < 2 region, as well as possible miscibility gaps among the phases. To support the experimental work, we performed computational modeling, first applying cluster expansion CE) coupled with relaxation using molecular dynamics. The stable phases found using the CE were then subjected to more rigorous density functional theory (DFT) calculations, to determine the most stable phases in the USi–USi2 phase space. We subsequently performed a detailed analysis of the dependence of the U–Si convex hull on values of the on-site Coulomb correction, Ueff, (DFT + U) using the U-ramping method. This information allowed us to study the influence of electron correlation, and to find Ueff values that best represents the U–Si convex hull. Lastly, the temperature dependence was calculated using determined Gibbs energies of the phases. In the USi–USi2 phase space, two stable phases were observed up to 1200 °C, with a possible hexagonal to tetragonal phase transition at 580 °C for one of the phases.

Vibrational properties of uranium fluorides

Multiphase mixtures of the uranium fluoride compounds UFx with x = 3, 4, 4.5, 5, whose local U–F bonding geometry is conserved, may result from UF6 reduction. One method for identifying multiphase mixtures is optical vibrational spectroscopy, but experimental Raman and IR spectra for UFx compounds are difficult to obtain. To supplement those experimental measurements, we use density functional perturbation theory (DFPT) with the on-site Coulomb correction (Hubbard + U) to calculate the phonon normal modes, their IR intensity, and their Raman activity for UF3, UF4, U2F9, and UF5. In addition, we use neutron spectroscopy to measure the phonon density of states of the most stable UF4. Our measurements on the Wide-Angular Range Chopper Spectrometer at the Spallation Neutron Source indicate that UF4 has a broad phonon spectrum centered at about 30 meV, but modeling of this spectrum indicates significant multiphonon scattering is present. DFPT calculations for U2F9, which shares structural features with UF4 and UF5, suggest a dynamic instability and we speculate that previous crystallographic measurements of this compound were possible only in multiphase mixtures. Analysis of the detailed atomic motions of phonons in UF5 indicate that a series of high energy modes (between 69 and 75 meV) are described by motion of single-coordinated F atoms (bonded to only one U atom). These single-coordinated F atoms are unique to the UF5 structure and those modes could be used to distinguish UF5 in a multiphase mixture using optical spectroscopy.