Structural,elastic, electronic and magnetic properties of perovskite-like Co3WC,Rh3WC and Ir3WC from first principles calculations

We have performed ab initio total energy calculations using the full-potential linearized augmented plane wave method (FP-LAPW) with the generalized gradient approximation (GGA) for the exchange-correlation potential to predict the structural, elastic, cohesive, electronic and magnetic properties of perovskite-like phases Co₃WC, Rh₃WC and Ir₃WC. The optimized lattice parameters, density, independent elastic constants (C_ij), bulk moduli, shear moduli, tetragonal shear moduli, compressibility, and Cauchy pressure, as well as electronic densities of states, cohesive and formation energies, atomic magnetic moments have been obtained and analyzed for the first time.     

The role of errors related to DFT methods in calculations involving ion pairs of ionic liquids

Ionic liquids (ILs) play a key role in many chemical applications. As regards the theoretical approach, ILs show added difficulties in calculations due to the composition of the ion pair and to the fact that they are liquids. Although density functional theory (DFT) can treat this kind of systems to predict physico–chemical properties, common versions of these methods fail to perform accurate predictions of geometries, interaction energies, dipole moments, and other properties related to the molecular structure. In these cases, dispersion and self‐interaction error (SIE) corrections need to be introduced to improve DFT calculations involving ILs. We show that the inclusion of dispersion is needed to obtain good geometries and accurate interaction energies. SIE needs to be corrected to describe the charges and dipoles in the ion pair correctly. The use of range–separated functionals allows us to obtain interaction energies close to the CCSD(T) level. © 2017 Wiley Periodicals, Inc.     

The first-principles treatment of the electron-correlation and spin-orbital effects in uranium mononitride nuclear fuels

The DFT+U calculations were employed in a detailed study of the strong electron correlation effects in a promising nuclear fuel-uranium mononitride (UN). A simple method for solving the multiple minima problem in DFT+U simulations and insure obtaining the correct ground state is suggested and applied. The crucial role of spin-orbit interactions in reproduction of the U atom total magnetic moment is demonstrated. Basic material properties (the lattice constants, the spin- and total magnetic moments on U atoms, the magnetic ordering, and the density of states) were calculated varying the Hubbard U-parameter. By varying the tetragonal unit cell distortion, the meta-stable states have been carefully identified and analyzed. The difference in the magnetic and structural properties obtained for the meta-stable and ground states is discussed. The optimal effective Hubbard parameter U(eff) = 1.85 eV reproduces correctly the UN anti-ferromagnetic ordering, and only slightly overestimates the experimental total magnetic moment of the U atom and the unit cell volume.     

Electronic properties of 3d transition metal dihalide monolayers predicted by DFT methods: Is there a pattern or are the results random?

We use periodic DFT calculations at LDA and PBE level to investigate 3d transition metal dihalide (TMDH) monolayers in H- and T-phase. By analyzing the phonon dispersion, we have obtained a rough overview which combinations may form stable structures. We have focused on identifying and explaining trends in the predicted electronic properties. Although their geometric structures are simple, the associated electronic and magnetic properties are not as easy to understand. At first glance, it seems that there is no clear trend, as even isovalence-electronic TMDH monolayers formed from the same metal but different halides can feature different magnetic moments. The identification of potential trends is further complicated by the fact that for a significant number of species, LDA results and PBE results predict different ground-state electronic structures. By rigorously analyzing the potential energy surfaces associated with different magnetic moments, we could show that the apparent inconsistencies can be easily understood as a result of the differences in the relative energy between electronic states of different magnetic moments. We further show that the trends in the band gaps can be easily rationalized by an electron counting rule based on simple symmetry arguments.     

Density functional theory and CCSD(T) evaluation of ionization potentials,redox potentials,and bond energies related to zirconocene polymerization catalysts

Quantum-mechanical-based computational design of molecular catalysts requires accurate and fast electronic structure calculations to determine and predict properties of transition-metal complexes. For Zr-based molecular complexes related to polyethylene catalysis, previous evaluation of density functional theory (DFT) and wavefunction methods only examined oxides and halides or select reaction barrier heights. In this work, we evaluate the performance of DFT against experimental redox potentials and bond dissociation enthalpies (BDEs) for zirconocene complexes directly relevant to ethylene polymerization catalysis. We also examined the ability of DFT to compute the fourth atomic ionization potential of zirconium and the effect the basis set selection has on the ionization potential computed with CCSD(T). Generally, the atomic ionization potential and redox potentials are very well reproduced by DFT, but we discovered relatively large deviations of DFT-calculated BDEs compared to experiment. However, evaluation of BDEs with CCSD(T) suggests that experimental values should be revisited, and our CCSD(T) values should be taken as most accurate.     

Density functional calculations of NMR chemical shifts and ESR g-tensors

An overview is given on recent advances of density functional theory (DFT) as applied to the calculation of nuclear magnetic resonance (NMR) chemical shifts and electron spin resonance (ESR) g-tensors. This is a new research area that has seen tremendous progress and success recently; we try to present some of these developments. DFT accounts for correlation effects efficiently. Therefore, it is the only first-principle method that can handle NMR calculations on large systems like transition-metal complexes. Relativistic effects become important for heavier element compounds; here we show how they can be accounted for. The ESR g-tensor is related conceptually to the NMR shielding, and results of g-tensor calculations are presented. DFT has been very successful in its application to magnetic properties, for metal complexes in particular. However, there are still certain shortcomings and limitations, e.g., in the exchange-correlation functional, that are discussed as well. Received: 24 October 1997 / Accepted: 19 December 1997     

Hybrid density functional theory band structure engineering in hematite

We present a hybrid density functional theory (DFT) study of doping effects in α-Fe(2)O(3), hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure α-Fe(2)O(3) crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.     

Structure simulation of ultrathin dichloromethane layer on a solid substrate by density functional theory and molecular dynamics simulations

The method for prediction of structural properties of ultrathin liquid layers has been developed on the base of the atomistic molecular dynamics (AMD) and the density functional theory (DFT). A comparative analysis of ultrathin dichloromethane layer density profiles on three types of solid flat substrates showed that these approaches can be effectively used as mutually complementary procedures to describe the structural properties of nanometer scale surface layers. We used AMD calculations to predict the dichloromethane layer density profile on a solid substrate. However, it is difficult and computationally expensive to calculate structural and thermodynamic layers properties. At the same time, DFT can retain the microscopic details of macroscopic systems at the calculative cost significantly lower than that used in AMD. Therefore, in context of DFT, the substrate potential parameters are adjusted to reproduce AMD data. Thus, the obtained potential allows us to compute structural characteristics and, further, can be used to predict other physical properties of ultrathin films within the DFT framework. For instance, we calculated the coefficient of thermal expansion of dichloromethane in the case of three different substrates such as graphite, silicon oxide, and gold.     

Quantum mechanical modeling of Zn-based spinel oxides: Assessing the structural,vibrational, and electronic properties

The structural, electronic, and vibrational properties of two leading representatives of the Zn-based spinel oxides class, normal ZnX₂O₄ (X = Al, Ga, In) and inverse Zn₂MO₄ (M = Si, Ge, Sn) crystals, were investigated. In particular, density functional theory (DFT) was combined with different exchange-correlation functionals: B3LYP, HSE06, PBE0, and PBESol. Our calculations showed good agreement with the available experimental data, showing a mean percentage error close to 3% for structural parameters. For the electronic structure, the obtained HSE06 band-gap values overcome previous theoretical results, exhibiting a mean percentage error smaller than 10.0%. In particular, the vibrational properties identify the significant differences between normal and inverse spinel configurations, offering compelling evidence of a structure-property relationship for the investigated materials. Therefore, the combined results confirm that the range-separated HSE06 hybrid functional performs the best in spinel oxides. Despite some points that cannot be directly compared to experimental results, we expect that future experimental work can confirm our predictions, thus opening a new avenue for understanding the structural, electronic, and vibrational properties in spinel oxides.     

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.     

A computational study of lithium ketone enolate aggregation in the gas phase and in THF solution

The aggregation state of several lithium enolates were calculated in the gas phase and in THF solution by the B3LYP DFT and MP2 methods. The gas phase free energies of aggregate formation were underestimated by the DFT calculations, compared to those obtained by the G3MP2 method, although DFT did correctly predict the hexamer to be the major gas phase species. The DFT calculations correctly predicted the tetramer to be the major species in THF, while MP2 underestimated the stability of the tetramer relative to the dimer.     

Electronic structure of the organic half-metallic magnet: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl

An accurate full-potential density-functional method is used to study the magnetic and half-metallic properties in the pure organic materials: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl. The total and partial density of states and atomic spin magnetic moments are calculated and discussed. It is found that the unpaired electrons in this compound are localized in a molecular orbital constituted primarily of π^*(NO) orbital, and the main contribution of the spin magnetic moment comes from the NO free radicals. It is predicted that this compound is half-metallic magnet. It is also found that there exists ferromagnetic intermolecular interaction in the compound.     

Transition Metal Substitutions Induce Ferromagnetism in Bi2Te3

The possibilities of magnetism induced by transition-metal atoms substitution in Bi₂Te₃ system are investigated by ab initio calculations. The calculated results indicate that a transition-metal atom substitution for a Bi atom produces magnetic moments, which are due to the spin-polarization of transition-metal 3d electrons. The values of magnetic moments are 0.92, 1.97, 2.97, 4.04, and 4.98 μ_B for 4% Ti-, V-, Cr-, Mn-, and Fe-doped Bi₂Te₃ respectively. When substituting two transition-metal atoms, the characteristics of exchanging couple depend upon the distributions of the Bi atoms substituted. When two transitionmetal atoms substituting for Bi atoms locate at the sites of Bi1 and Bi5, with the distance of 11.52 Å, the Bi_1.84TM_0.16Te₃ system is energetically most stable and exhibits ferromagnetic coupling.     

Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields

We apply density functional theory (DFT) and the DFT+U technique to study the adsorption of transition metal porphine molecules on atomistically flat Au(111) surfaces. DFT calculations using the Perdew-Burke-Ernzerhof exchange correlation functional correctly predict the palladium porphine (PdP) low-spin ground state. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with experiments on substituted porphyrins. It exhibits no covalent bonding to Au(111), and the binding energy is a small fraction of an electronvolt. The DFT+U technique, parametrized to B3LYP-predicted spin state ordering of the Mn d-electrons, is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state. Its interaction with the gold substrate is stronger and more site-specific than that of PdP. The binding can be partially reversed by applying an electric potential, which leads to significant changes in the electronic and magnetic properties of adsorbed MnP and approximately 0.1 A changes in the Mn-nitrogen distances within the porphine macrocycle. We conjecture that this DFT+U approach may be a useful general method for modeling first-row transition metal ion complexes in a condensed-matter setting.     

First-Principles Study of Magnetism in Transition Metal Doped Na0.5Bi0.5TiO3 System

The origins of magnetism in transition-metal doped Na_0.5Bi_0.5TiO₃ system are investigated by ab initio calculations. The calculated results indicate that a transition-metal atom substitution for a Ti atom produces magnetic moments, which are due to the spin-polarization of transition-metal 3d electrons. The characteristics of exchange coupling are also calculated, which shows that in Cr-/Mn-/Fe-/Co-doped Na_0.5Bi_0.5TiO₃ system, the antiferromagnetic coupling is favorable. The results can successfully explain the experimental phenomenon that, in Mn-/Fe-doped Na_0.5Bi_0.5TiO₃ system, the ferromagnetism disappears at low temperature and the paramagnetic component becomes stronger with the increase of doping concentration of Mn/Fe/Co ions. Unexpectedly, we find the Na_0.5Bi_0.5Ti_0.67V_0.33iO₃ system with ferromagnetic coupling is favorable and produces a magnetic moment of 2.00 μB, which indicates that low temperature ferromagnetism materials could be made by introducing V atoms in Na_0.5Bi_0.5TiO₃. This may be a new way to produce low temperature multiferroic materials.     

On the electronic structure of transition-metal oxide cations:DFTCalculations for VO+ and MoO+

Two transition-metal oxide diatomic cations, VO⁺ and MoO⁺ are considered in this article. Ground- and excited-state properties of the cations are derived from spin-polarized DF calculations, including spectroscopic constants and metal–oxygen bonding features. A set of ionization potentials are calculated and, for vanadium oxide, compared with photoelectron spectroscopy data and a few available ab initio calculations. All calculated properties are close to experiment, the agreement being much better than for other traditional quantum chemical calculations. Present results together with our earlier findings for neutral molecules provide an excellent confirmation of the good performance of DFT in the case of transition-metal systems. © 1995 John Wiley & Sons, Inc.