Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps

Investigation of the electronic and magnetic properties of bare and oxygen-terminated ordered double transition-metal MXenes for spintronic applications

MXenes are a large and new family of intrinsically magnetic two-dimensional (2D) transition-metal carbides and nitrides. This family has been adding new members since their first discovery in 2011, and has expanded with the exploring of ordered double transition-metal (DTM) MXenes. In this study, we have investigated the electronic and magnetic properties of thirteen bare and fourteen oxygen-terminated DTM MXene structures (M3C2, M3C2O2, MM'C2 and MM'C2O2, M = Ti, Zr, Cr, and Mo; M' = Ti, V, Nb, and Ta). The Hubbard-U parameter strongly depends on the atom environment and the coordination number in the cell. Therefore, for the first time in the literature, we have calculated the Hubbard-U parameters for each considered MXene structure systematically instead of taking them randomly. The investigated MXene structures have striking properties with respect to their magnetic ground states, and show ferromagnetic to antiferromagnetic or non-magnetic properties, accompanied by semiconductor to metallic or semi-metallic properties, depending on the transition metal(s) or termination by oxygen. We have performed Monte Carlo simulations to obtain the magnetic phase transition temperature of each structure. Additionally, coercivity and remanence values have been calculated for ferromagnetic cases, and we have investigated the hysteresis features of the MXenes of interest by applying a cyclic magnetic field at several temperatures.

A systematic DFT study of structure and electronic properties of titanium dioxide

DFT functionals are of paramount importance for an accurate electronic and structural description of transition metal systems. In this work, a systematic analysis using some well-known and commonly used DFT functionals is performed. A comparison of the structural and energetic parameters calculated with the available experimental data is made in order to find the adequate functional for an accurate description of the TiO2 bulk and surface of both anatase and rutile structures. In the absence of experimental data on the surface energy, the theoretical predictions obtained using the high-accuracy HSE06 functional were used as a reference to compare against the surface energy values calculated with the other DFT functionals. A clear improvement in the electronic description of both anatase and rutile was observed by introducing the Hubbard U correction term to PBE, PW91, and OptPBE functionals. The OptPBE-U4 functional was found to offer a good compromise between accurately describing the structural and electronic properties of titania.

Roles of Impurity Levels in 3d Transition Metal-Doped Two-Dimensional Ga2O3

Doping engineering is crucial for both fundamental science and emerging applications. While transition metal (TM) dopants exhibit considerable advantages in the tuning of magnetism and conductivity in bulk Ga2O3, investigations on TM-doped two-dimensional (2D) Ga2O3 are scarce, both theoretically and experimentally. In this study, the detailed variations in impurity levels within 3d TM-doped 2D Ga2O3 systems have been explored via first-principles calculations using the generalized gradient approximation (GGA) +U method. Our results show that the Co impurity tends to incorporate on the tetrahedral GaII site, while the other dopants favor square pyramidal GaI sites in 2D Ga2O3. Moreover, Sc3+, Ti4+, V4+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+, Cu2+, and Zn2+ are the energetically favorable charge states. Importantly, a transition from n-type to p-type conductivity occurs at the threshold Cu element as determined by the defect formation energies and partial density of states (PDOS), which can be ascribed to the shift from electron doping to hole doping with respect to the increase in the atomic number in the 3d TM group. Moreover, the spin configurations in the presence of the square pyramidal and tetrahedral coordinated crystal field effects are investigated in detail, and a transition from high-spin to low-spin arrangement is observed. As the atomic number of the 3d TM dopant increases, the percentage contribution of O ions to the total magnetic moment significantly increases due to the electronegativity effect. Additionally, the formed 3d bands for most TM dopants are located near the Fermi level, which can be of significant benefit to the transformation of the absorbing region from ultraviolet to visible/infrared light. Our results provide theoretical guidance for designing 2D Ga2O3 towards optoelectronic and spintronic applications.

Orbital-Resolved DFT+U for Molecules and Solids

We present an orbital-resolved extension of the Hubbard U correction to density-functional theory (DFT). Compared to the conventional shell-averaged approach, the prediction of energetic, electronic and structural properties is strongly improved, particularly for compounds characterized by both localized and hybridized states in the Hubbard manifold. The numerical values of all Hubbard parameters are readily obtained from linear-response calculations. The relevance of this more refined approach is showcased by its application to bulk solids pyrite (FeS2) and pyrolusite (β-MnO2), as well as to six Fe(II) molecular complexes. Our findings indicate that a careful definition of Hubbard manifolds is indispensable for extending the applicability of DFT+U beyond its current boundaries. The present orbital-resolved scheme aims to provide a computationally undemanding yet accurate tool for electronic structure calculations of charge-transfer insulators, transition-metal (TM) complexes and other compounds displaying significant orbital hybridization.

High-entropy engineering of the crystal and electronic structures in a Dirac material

Dirac and Weyl semimetals are a central topic of contemporary condensed matter physics, and the discovery of new compounds with Dirac/Weyl electronic states is crucial to the advancement of topological materials and quantum technologies. Here we show a widely applicable strategy that uses high configuration entropy to engineer relativistic electronic states. We take the AMnSb2 (A = Ba, Sr, Ca, Eu, and Yb) Dirac material family as an example and demonstrate that mixing of Ba, Sr, Ca, Eu and Yb at the A site generates the compound (Ba0.38Sr0.14Ca0.16Eu0.16Yb0.16)MnSb2 (denoted as A5MnSb2), giving access to a polar structure with a space group that is not present in any of the parent compounds. A5MnSb2 is an entropy-stabilized phase that preserves its linear band dispersion despite considerable lattice disorder. Although both A5MnSb2 and AMnSb2 have quasi-two-dimensional crystal structures, the two-dimensional Dirac states in the pristine AMnSb2 evolve into a highly anisotropic quasi-three-dimensional Dirac state triggered by local structure distortions in the high-entropy phase, which is revealed by Shubnikov-de Haas oscillations measurements.

First-Principles Calculations of Magnetite (Fe3O4) above the Verwey Temperature by Using Self-Consistent DFT + U + V

In this report, we have used the DFT + U + V approach, an extension of the DFT + U approach that takes into account both on-site and intersite interactions, to simulate structural, magnetic, and electronic properties together with the Fe and O K-edge XAS spectra of Fe3O4 above the Verwey temperature (Tv). Moreover, we compared the simulated XAS spectra with experimental XAS data. We examined both orthogonalized and nonorthogonalized atomic orbital projectors and compared DFT + U + V to DFT, DFT + U, and HSE as a hybrid functional. It is noteworthy that, despite the widespread use of the same Hubbard U value for Feoct and Fetet at the DFT + U level in the literature, the HP code identified two distinct values for them using the Hubbard approaches (DFT + U and DFT + U + V). The resulting Hubbard U and V parameters are strongly dependent on the chosen orbital projectors. This study demonstrates how DFT + U + V can improve the structural, magnetic, and electronic properties of Fe3O4 compared to approximate DFT and DFT + U. In this context, DFT + U + V supports the half-metallic character of the bulk crystal Fe3O4 above Tv, since the Fermi level is found in the t2g band with a Feoct down-spin. Thus, the observations in the current study emphasize the significance of intersite interactions in the theoretical analysis of Fe3O4 above the Tv.

Machine Learning-Accelerated Discovery of A2BC2 Ternary Electrides with Diverse Anionic Electron Densities

This study combines machine learning (ML) and high-throughput calculations to uncover new ternary electrides in the A2BC2 family of compounds with the P4/mbm space group. Starting from a library of 214 known A2BC2 phases, density functional theory calculations were used to compute the maximum value of the electron localization function, indicating that 42 are potential electrides. A model was then trained on this data set and used to predict the electride behavior of 14,437 hypothetical compounds generated by structural prototyping. Then, the stability and electride features of the 1254 electride candidates predicted by the model were carefully checked by high-throughput calculations. Through this tiered approach, 41 stable and 104 metastable new A2BC2 electrides were predicted. Interestingly, all three kinds of electrides, i.e., electron-deficient, electron-neutral, and electron-rich electrides, are present in the set of predicted compounds. Three of the most promising new electrides (two electron-rich, Nd2ScSi2 and La2YbGe2, and one electron-deficient Y2LiSi2) were then successfully synthesized and characterized experimentally. Furthermore, the synthesized electrides were found to exhibit high catalytic activities for NH3 synthesis under mild conditions when Ru-loaded. The electron-deficient Y2LiSi2, in particular, was seen to exhibit a good balance of catalytic activity and chemical stability, suggesting its future application in catalysis.

Stabilizing tetramethylammonium lead iodide perovskite and exploring its electronic and optical absorption for solar cell absorber application

Modeling perovskites as solar cell absorbers has become popular due to the breakthrough of methylammonium lead iodide (CH3NH3PbI3). In this study, we modeled a tetramethylammonium lead iodide (CH3)4NPbI3 structure. We further confirmed the stability of the structure by determining the phonon dispersion using density functional perturbation theory. We calculated the spin-orbit and non-spin-orbit coupling-based electronic structure using the Perdew-Burke-Ernzerhof exchange-correlation functional within the generalized gradient approximation of the density functional theory and the self-consistent GW quasiparticle methods. Similarly, the absorption spectra were calculated from the real and imaginary parts of the dielectric tensor obtained from solving the Bethe-Salpeter equation using the GW quasiparticle database. The solar cell absorber spectroscopic limited maximum efficiency was calculated at 293.15 K. The self-consistent GW method without spin-orbit coupling reported bandgaps of 2.63 eV and 2.89 eV for GW0 and GW methods, respectively, in agreement with experimental reports. The phonon dispersion showed positive phonon modes across the high symmetry point, which attest to its thermodynamic stability. The absorption coefficient on the order of 105 was reported along the ultraviolet region. The standard limited maximum efficiency between 7% and 12% was recorded at 293.15 K between 0.01 and 100 μm absorber thicknesses. The thermodynamic stability, high absorption coefficient, and low transmittance indicated exciting prospects for a non-transparent (CH3)4NPbI3 solar cell absorber.

Doped MoS2 Polymorph for an Improved Hydrogen Evolution Reaction

Green hydrogen produced from solar energy could be one of the solutions to the growing energy shortage as non-renewable energy sources are phased out. However, the current catalyst materials used for photocatalytic water splitting (PWS) cannot compete with other renewable technologies when it comes to efficiency and production cost. Transition-metal dichalcogenides, such as molybdenum disulfides (MoS₂), have previously proven to have electronic and optical properties that could tackle these challenges. In this work, optical properties, the d-band center, and Gibbs free energy are calculated for seven MoS₂ polymorphs using first-principles calculations and density functional theory (DFT) to show that they could be suitable as photocatalysts for PWS. Out of the seven, the two polymorphs 3H_a and 2R₁ were shown to have d-band center values closest to the optimal value, while the Gibbs free energy for all seven polymorphs was within 5% of each other. In a previous study, we found that 3H_b had the highest electron mobility among all seven polymorphs and an optimal bandgap for photocatalytic reactions. The 3H_b polymorphs were therefore selected for further study. An in-depth analysis of the enhancement of the electronic properties and the Gibbs free energy through substitutional doping with Al, Co, N, and Ni was carried out. For the very first time, substitutional doping of MoS₂ was attempted. We found that replacing one Mo atom with Al, Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10, which would increase the hydrogen evolution reaction of the catalyst. Our study further shows that 3H_b with one S atom replaced with Al, Co, I, N, or Ni is dynamically and mechanically stable, while for 3H_b, replacing one Mo atom with Al and Ni makes the structure stable. Based on the low Gibbs free energy, stability, and electronic bandgap 3H_b, MoS₂ doped with Al for one Mo atom emerges as a promising candidate for photocatalytic water splitting.

Colloidal 2D Lead Chalcogenide Nanocrystals: Synthetic Strategies, Optical Properties, and Applications

Lead chalcogenide nanocrystals (NCs) are an emerging class of photoactive materials that have become a versatile tool for fabricating new generation photonics devices operating in the near-IR spectral range. NCs are presented in a wide variety of forms and sizes, each of which has its own unique features. Here, we discuss colloidal lead chalcogenide NCs in which one dimension is much smaller than the others, i.e., two-dimensional (2D) NCs. The purpose of this review is to present a complete picture of today's progress on such materials. The topic is quite complicated, as a variety of synthetic approaches result in NCs with different thicknesses and lateral sizes, which dramatically change the NCs photophysical properties. The recent advances highlighted in this review demonstrate lead chalcogenide 2D NCs as promising materials for breakthrough developments. We summarized and organized the known data, including theoretical works, to highlight the most important 2D NC features and give the basis for their interpretation.

An open database of computed bulk ternary transition metal dichalcogenides

We present a dataset of structural relaxations of bulk ternary transition metal dichalcogenides (TMDs) computed via plane-wave density functional theory (DFT). We examined combinations of up to two chalcogenides with seven transition metals from groups 4-6 in octahedral (1T) or trigonal prismatic (2H) coordination. The full dataset consists of 672 unique stoichiometries, with a total of 50,337 individual configurations generated during structural relaxation. Our motivations for building this dataset are (1) to develop a training set for the generation of machine and deep learning models and (2) to obtain structural minima over a range of stoichiometries to support future electronic analyses. We provide the dataset as individual VASP xml files as well as all configurations encountered during relaxations collated into an ASE database with the corresponding total energy and atomic forces. In this report, we discuss the dataset in more detail and highlight interesting structural and electronic features of the relaxed structures.

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 Li_xMn₂O₄ and Li_xMn_1.5Ni_0.5O₄ 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+U+V with on-site U and inter-site V 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 U is crucial to obtain the right trends in properties of these materials, V 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.

First-principles calculations of hematite (α-Fe2O3) by self-consistent DFT+U+V

Owing to the confined Fe-3d orbitals and self-interaction error of exchange-correlation functionals, approximate DFT fails to describe iron oxides electronic structure and magnetic properties accurately. Hybrid DFT or DFT + U can solve these problems, but the former is expensive, and the latter only considers on-site interactions. Here, we used DFT + U + V, a DFT + U extension including inter-site interactions, to simulate the structural, magnetic, and electronic properties, along with Fe and O K-edge XAS spectra of α-Fe₂O₃. Two types of atomic orbital projectors were studied, orthogonalized and non-orthogonalized. DFT + U + V improves the description of the structural, magnetic, and electronic properties of α-Fe₂O₃ compared to approximate DFT. The accuracy of the correction depends on the orbital projector used. DFT + U + V with orthogonalized projectors achieves the best experimental agreement at a fraction of hybrid DFT cost. This work emphasizes the importance of inter-site interactions and the type of atomic orbital projectors used in the theoretical research of α-Fe₂O₃.

Ab initio methods for the computation of physical properties and performance parameters of electrochemical energy storage devices

With the rapid development of electric vehicles and mobile technologies, there is a high demand for electrochemical energy storage devices and electrochemical energy conversion devices. Devices meeting these needs include metal-ion batteries (MIBs), supercapacitors (SCs), electrochromic devices (ECDs), and multifunctional devices such as electrochromic batteries and supercapatteries. Currently, the goal has been the enhancement of operational parameters and physical properties that results in a higher performance of these devices. In the case of batteries, SCs, and supercapatteries, scientists seek to improve the equilibrium voltage, energy density, power, capacitance, and charge rate. In the case of ECDs, the focus is on improvement of the optical modulation and coloration efficiency. However, synthesis and characterization of new materials, or of materials with optimized properties, is time consuming and highly expensive. Computational simulation of materials can expedite the experimental endeavor by modelling novel atomic structures and predicting device performance. This is possible using ab initio theories and applying physical principles that allow us to understand the underlying mechanisms governing the behavior of materials in these devices. Taking as a point of departure density functional theory (DFT), in this review, we discuss the first principles methods used for the computation of physical properties and performance parameters of electrochemical energy storage devices. A wide coverage of DFT is given, dealing with the strengths and weaknesses of the most popular functionals used in the field of electrochemical energy storage. With these tools, ab initio methods for the computation of basic properties such as effective mass, mobility, optical band gap, transmissivity, conductivity (ionic and electronic), and criteria for structure stability (cohesive energy, formation energy, adsorption energy, and phonon frequency) are addressed. We also highlight the first principles techniques for the calculation of performance parameters in MIBs, SCs, and ECDs.

Ab Initio Study of Carrier Mobility, Thermodynamic and Thermoelectric Properties of Kesterite Cu2ZnGeS4

The kesterite Cu2ZnGeS4 (CZGS) has recently gained significant interest in the scientific community. In this work, we investigated the thermodynamic and thermoelectric properties of CZGS by employing the first-principals calculation in association with the quasi-harmonic approximation, Boltzmann transport theory, deformation potential theory, and slack model. We obtained a bandgap of 2.05 eV and high carrier mobility. We found that CZGS exhibits adequate thermoelectric properties as a promising material for thermoelectric applications. The calculated Seebeck coefficient at room temperature is 149 µV·K-1. We also determined the thermal and electrical conductivity, the power factor, and the figure of merit. In addition, the thermodynamic properties such as Debye temperature, entropy, and constant volume heat capacity are estimated. According to our results, it is concluded that the Slack model fails to provide correct values for lattice thermal conductivity in this material.

TiO2 Gas Sensors Combining Experimental and DFT Calculations: A Review

Gas sensors play an irreplaceable role in industry and life. Different types of gas sensors, including metal-oxide sensors, are developed for different scenarios. Titanium dioxide is widely used in dyes, photocatalysis, and other fields by virtue of its nontoxic and nonhazardous properties, and excellent performance. Additionally, researchers are continuously exploring applications in other fields, such as gas sensors and batteries. The preparation methods include deposition, magnetron sputtering, and electrostatic spinning. As researchers continue to study sensors with the help of modern computers, microcosm simulations have been implemented, opening up new possibilities for research. The combination of simulation and calculation will help us to better grasp the reaction mechanisms, improve the design of gas sensor materials, and better respond to different gas environments. In this paper, the experimental and computational aspects of TiO2 are reviewed, and the future research directions are described.

Koopmans Spectral Functionals in Periodic Boundary Conditions

Koopmans spectral functionals aim to describe simultaneously ground-state properties and charged excitations of atoms, molecules, nanostructures, and periodic crystals. This is achieved by augmenting standard density functionals with simple but physically motivated orbital-density-dependent corrections. These corrections act on a set of localized orbitals that, in periodic systems, resemble maximally localized Wannier functions. At variance with the original, direct supercell implementation (Phys. Rev. X 2018, 8, 021051), we discuss here (i) the complex but efficient formalism required for a periodic boundary code using explicit Brillouin zone sampling and (ii) the calculation of the screened Koopmans corrections with density functional perturbation theory. In addition to delivering improved scaling with system size, the present development makes the calculation of band structures with Koopmans functionals straightforward. The implementation in the open-source Quantum ESPRESSO distribution and the application to prototypical insulating and semiconducting systems are presented and discussed.

Pressure-Induced Enhanced Optical Absorption in Sulvanite Compound Cu3TaX4 (X = S, Se, and Te): An ab Initio Study

Ab initio study on the family of ternary copper chalcogenides Cu₃TaX₄ (X = S, Se, and Te) is performed to investigate the suitability of these compounds to applications as photovoltaic absorber materials. The density functional theory based full potential linearized augmented plane wave method (FP-LAPW method) is employed for computational purposes. The electronic structure and optical properties are determined including electron-electron interaction and spin-orbit coupling (SOC), within the generalized gradient approximation plus Hubbard U (GGA+U) and GGA+U+SOC approximation. The large optical band gaps of Cu₃TaS₄ and Cu₃TaSe₄ considered ineffective for absorber materials, and also the hole effective mass has been modulated through applied pressure. These materials show extreme resistance to external pressure, and are found to be stable up to a pressure range of 10 GPa, investigated using phonon dispersion calculations. The observed optical properties and the absorption coefficients within the visible-light spectrum make these compounds promising materials for photovoltaic applications. The calculated energy and optical band gaps are consistent with the available literature and are compared with the experimental results where available.

Elucidating Synergistic Mechanisms of Adsorption and Electrocatalysis of Polysulfides on Double-Transition Metal MXenes for Na-S Batteries

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na₂S_n) in electrolytes, and the slower kinetics for the decomposition of solid Na₂S, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo₂TiC₂T₂, (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo₂TiC₂S₂ and Mo₂TiC₂O₂ as AMs by analyzing their interactions with S₈/Na₂S_n (n = 1, 2, 4, 6, and 8). Mo₂TiC₂S₂ provides moderate adsorption strength compared to Mo₂TiC₂O₂, therefore, it is expected to effectively inhibit Na₂S_n dissolution and shuttling without causing decomposition of Na₂S_n. The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo₂TiC₂T₂ during discharge. Additionally, both Mo₂TiC₂S₂ and Mo₂TiC₂O₂ demonstrated effective electrocatalytic activity for the decomposition of Na₂S, with a substantial reduction in the energy barrier to 1.59 eV for Mo₂TiC₂S₂ and 1.67 eV for Mo₂TiC₂O₂. While Mo₂TiC₂O₂ had superior binding properties, structural distortion is observed in Na₂S_n, which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo₂TiC₂S₂ can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.

Effect of Doping on the Electronic Structure of the Earth’s Lower Mantle Compounds: FeXO3 with X = C, Al, Si

The effect of the mutual doping of C, Si, and Al atoms on the electronic structure and magnetic properties of FeXO₃ (X = C, Al, Si) compounds, which are constituent compounds of the Earth’s lower mantle, was studied. In our first principles calculations, it was found that doping with carbon for both FeSiO₃ and FeAlO₃ leads to the transition of the compound from a half-metallic state to a metallic one. The values of the magnetic moments of Fe were obtained for pure and doped compounds. For the doped compounds, there is a tendency of the Fe magnetic moment to increase with the growth in the number of substituted ions in the case of replacing Si with C and Si for Al; on the contrary, in the case of replacing Al with C and Si, a decrease in the magnetic moment was revealed. For FeXO₃ (X = C, Al, Si), the obtained magnetic moment values were found to be in a good agreement with the known experimental data.

Structural Properties and Magnetic Ground States of 100 Binary d-Metal Oxides Studied by Hybrid Density Functional Methods

d-metal oxides play a crucial role in numerous technological applications and show a great variety of magnetic properties. We have systematically investigated the structural properties, magnetic ground states, and fundamental electronic properties of 100 binary d-metal oxides using hybrid density functional methods and localized basis sets composed of Gaussian-type functions. The calculated properties are compared with experimental information in all cases where experimental data are available. The used PBE0 hybrid density functional method describes the structural properties of the studied d-metal oxides well, except in the case of molecular oxides with weak intermolecular forces between the molecular units. Empirical D3 dispersion correction does not improve the structural description of the molecular oxides. We provide a database of optimized geometries and magnetic ground states to facilitate future studies on the more complex properties of the binary d-metal oxides.

Effects of the Hubbard U correction on the electronic and magnetic properties of the tetragonal V2P2 sheet

A recent theoretical work predicted the orthorhombic phase of the V2P2 sheet with the half-metallic electronic property using a linear combination of atomic orbitals (LCAO) basis set based on density functional theory (DFT). However, in the plane-wave DFT method, the tetragonal (t) V2P2 phase is the ground state structure. The total energy of the optimized tetragonal V2P2 is 0.91 eV per cell lower than that of the orthorhombic phase. Herein, we investigated the effects of Hubbard U correction onthe electronic, magnetic, and adsorption properties of the t-V2P2 sheet. The t-V2P2 sheet is found to be dynamically and mechanically stable. The t-V2P2 sheet prefers an antiferromagnetic ground state with an indirect narrowed bandgap of 0.23 eV. The estimated electron mobility in the t-V2P2 sheet at room temperature is approximately 24 times that of a hole. The t-V2P2 sheet exhibits a sizable magnetic anisotropy (MAE) of 69.63 μeV per V atom with in-plane magnetization. Mean-field approximation based on the 2D classical Heisenberg model predicts a high Néel temperature (T N) of the t-V2P2 sheet up to 1263 K. The Li atom adsorption on the t-V2P2 sheet shows a transition from semiconductor to metal. Also the Li-V2P2 system has a residual integer magnetic moment of 1 μ B. Due to strong steric coulomb repulsion, the minimum diffusion energy barrier (E a) for the Li-ion on the t-V2P2 surface is high enough to make the Li atom immobile. Our findings demonstrate the potential of the t-V2P2 sheet for antiferromagnetic spintronics and sensing applications.

Evaluation of optical band gaps and dopant state energies in transition metal oxides using oxidation-state constrained density functional theory

We report the use of oxidation-state constrained density functional theory (OS-CDFT) to calculate the optical band gaps of transition metal oxides and dopant state energies of different doped anatase. OS-CDFT was used to control electron transfer from the valence band maximum of the transition metal system to the conduction band minimum or to the dopant state in order to calculate the band gap or the dopant state energies respectively. The calculation of the dopant state energies also allows identification of the transition responsible for the reduced band gap of the doped system in ambiguous cases. We applied this approach to the band gap calculation in TiO₂anatase and rutile, vanadium pentoxide (V₂O₅), chromium(III) oxide (Cr₂O₃), manganese(IV) oxide (MnO₂), ferric oxide (Fe₂O₃), ferrous oxide (FeO) and cobalt(II) oxide (CoO). The dopant state energies calculations were carried out in the V-, Cr-, Mn-, and Fe-doped anatase.