Hybrid Hartree Research Articles

Frontier orbitals and quasiparticle energy levels in ionic liquids

Ionic liquids play an important role in many technological applications and a detailed understanding of their frontier molecular orbitals is required to optimize interfacial barriers, reactivity and stability with respect to electron injection and removal. In this work, we calculate quasiparticle energy levels of ionic liquids using first-principles many-body perturbation theory within the GW approximation and compare our results to various mean-field approaches, including semilocal and hybrid density-functional theory and Hartree–Fock. We find that the mean-field results depend qualitatively and quantitatively on the treatment of exchange–correlation effects, while GW calculations produce results that are in excellent agreement with experimental photoelectron spectra of gas phase ion pairs and ionic liquids. These results establish the GW approach as a valuable tool for understanding the electronic structures of ionic liquids.

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations.

Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS3 ) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree-Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ∼15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, the collapse of the band gap occurs due to changes in the electronic structure of FePS3 induced by relative displacements of phosphorus or sulfur atoms along the c-axis direction under pressure.

On the Reduction of O2 on Cathode Surfaces of Co–Corrin and Co–Porphyrin: A Computational and Experimental Study on Their Relative Efficiencies in H2O/H2O2 Formation

The mechanisms for O2 reduction and H2O/H2O2 formation on Co–corrin and Co–porphyrin cathode surfaces of the proton exchange membrane fuel cell (PEMFC) systems have been studied by hybrid Hartree–F...

Simple Ethanol Refluxing Method for Production of Blue-Colored Titanium Dioxide with Oxygen Vacancies and Visible Light-Driven Photocatalytic Properties

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2–x) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band—responsible for the blue coloration—extending from the green region (∼2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree–Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (∼2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.

First-Principles Evaluation of the Morphology of WS2 Nanotubes for Application as Visible-Light-Driven Water-Splitting Photocatalysts.

One-dimensional tungsten disulfide (WS2) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters dNT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV < Δεgap < 2.6 eV). We have simulated the electronic structure of nanotubes with diameters up to 12.0 nm. The calculated top of the valence band and the bottom of the conduction band (εVB and εCB, respectively) have been properly aligned relatively to the oxidation (εO2/H2O) and reduction (εH2/H2O) potentials of water. Very narrow nanotubes (0.5 < dNT < 1.9 nm) are unsuitable for water splitting because the condition εVB < εO2/H2O < εH2/H2O < εCB does not hold. For nanotubes with dNT > 1.9 nm, the condition εVB < εO2/H2O < εH2/H2O < εCB is fulfilled. The values of εVB and εCB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree–Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)@(n, n) and (m, 0)@(n, 0) as well as triple-walled (l, l)@(m, m)@(n, n) and (l, 0)@(m, 0)@(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS2 NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap.

The effects of non-hydrostatic stress on the structure and properties of alpha-quartz

The study of the effects of non-hydrostatic stresses on rock-forming minerals is fundamental for understanding how minerals respond to tectonic stresses in the Earth’s interior. Larger deviatoric stresses of the order of GPa can arise from physical interaction between minerals (e.g. host–inclusion systems, compositional gradients) at the micrometric scale, even if at the large-scale large stresses are annealed by plastic flows. Thus, experimental data under hydrostatic pressure do not provide us access to all the possible modifications and structural changes experienced by minerals subjected to deviatoric stresses. Therefore, we carried out ab initio hybrid Hartree–Fock/Density Functional Theory simulations to determine the properties of alpha-quartz, since it is one of the most abundant minerals in the Earth’s crust with a very simple chemistry and structure. We calculated its structure, elastic parameters and Raman-active vibrational modes as a function of different applied strains, which allow us to show how phonon frequencies and structural parameters (bond lengths and angles) are affected by deviatoric stress conditions rather than hydrostatic pressure.

The electronic structure and the formation of polarons in Mo-doped BiVO4 measured by angle-resolved photoemission spectroscopy.

We experimentally investigated the electronic structure of Mo-doped BiVO4 high-quality single-crystals with synchrotron radiation-excited angle-resolved photoelectron spectroscopy (ARPES). By photon-energy dependent ARPES, we measured the bulk-derived valence band dispersion along the direction normal to the (010) cleavage plane, while the dispersion along the in-plane directions is obtained by angle-dependent measurements at fixed photon energy. Our data show that the valence band has a width of about 4.75 eV and is composed of many peaks, the two most intense have energies in good agreement with the theoretically calculated ones. A non-dispersive feature is observed in the fundamental gap, which we attribute to quasiparticle excitations coupling electrons and phonons, i.e. polarons. The determination of the polaron peak binding energy and bulk band gap allows to fix the value of the theoretical mixing parameter necessary in hybrid Hartree–Fock calculations to reproduce the experimental data. The attribution of the in-gap peak to polarons is strengthened by our discussion in the context of experimental transport data and ab initio theory.

Ab Initio Calculations on the Electronic Structure and Photocatalytic Properties of Two‐Dimensional WS2 (0001) Nanolayers of Varying Thickness

2D layered transition metal dichalcogenides (TMDC) structurized in hexagonal 2H c phase possess space group P63/mmc and graphite‐type morphology (e.g., WS2 bulk), as well as strong chemical bonds and weak coupling between S‐W‐S three‐plane monolayers (MLs). Ab initio calculations on pristine WS2 (0001) nanolayers are performed using CRYSTAL14 code within the formalism of hybrid density functional theory and Hartree–Fock method (HSE06 Hamiltonian properly adapted to describe tungsten disulphide bulk). An ordered stack of graphene‐type WS2 (0001)‐oriented MLs (in absence of any point defects including dopants) with thikness varies from 1 to 40 MLs is found to be suitable for photocatalysis since irrespectively of this thickness the bandgap corresponds to the varied range of visible spectrum between its violet and red edges, respectively. The authors calculate the electronic structure of these nanolayers. The highest photocatalytic efficiency is achieved for WS2 2 ML with band gap △ϵgap ≈ 2.06 eV (yellow range of the visible spectrum). Additionally, all the edges of ordered pristine 2H c WS2 (0001) nanosheets (ϵVB and ϵCB levels corresponding to the top of the valence band and the bottom of the conduction band) are properly aligned relative to the oxidation and reduction potentials ( and ): ϵVB &lt; &lt; &lt; ϵCB, which means that probability of photocatalytic electron‐hole recombination is rather minimal, being reduced with a decrease of stack thickness.

Ab Initio Calculations of the Main Crystal Surfaces of Baryte (BaSO4)

In this work, we revisited the theoretical morphology of baryte for determining the character (flat, stepped, and kinked) and the different profiles (or terminations) of the crystal faces, and presented an accurate ab initio study of the structures and surface energies at 0 K of the low-index (001), (011), (210), (101), (100), (010), (201), (012), and (211) baryte faces, by using the hybrid Hartree–Fock/density functional B3LYP Hamiltonian. According to the surface energy values, the stability order of the baryte faces was found to be (210) ≅ (001) < (211) < (101) < (010) < (011) < (201) ≅ (100) < (012). Then, the equilibrium shape of baryte was drawn and compared with the previous ones obtained at an empirical level. This is a preliminary work, which is necessary to successively evaluate the influence of water and/or impurities adsorption on the most important baryte surfaces, both at equilibrium and during growth. Indeed, to evaluate these adsorption effects on the crystal morphology, it is fundamental ...

Ab initio periodic modelling of the vibrational spectra of molecular crystals: the case of uracil

The structure and vibrational spectra of solid uracil have been simulated in the framework of Density Functional Theory (DFT) using a periodic unit cell model. Structural parameters are reproduced reasonably well by using the dispersion corrected, global hybrid Hartree–Fock/DFT functional B3LYP-D* and an all-electron, Gaussian type, triple zeta basis set with polarisation. The periodic calculation provides the full set of fundamental harmonic vibrational modes, whose nature can be investigated by inspecting the corresponding eigenvectors. Accounting for dispersive interactions indirectly affects the spectra, through the impact on the cell parameters. Marked differences are found between the gas and solid phase spectra, that can be related to either mode coupling or direct alteration of the potential energy via neighbour-neighbour molecular interactions. Anharmonicity needs to be considered for a meaningful comparison with experiments; a single scaling factor provides a significantly improved agreement for most of the frequencies, except for the NH stretchings, which require a larger downscaling. This rescaling strategy yields results of comparable quality with respect to previously reported calculations with a cluster model and a perturbative treatment of anharmonicity.

Unusual Metallic Multiferroic Transitions in Electron‐Doped PbTiO3

The coexistence of some materials which are normally in mutually exclusive states is attracting considerable attention as an intriguing means of obtaining nontrivial physical phenomena and unconventional multifunctional substances. Although single‐phase materials endowed with integrated ferroelectric, magnetic, and optical multifunctions hold promise for new technological paradigms, the mutually exclusive mechanisms within ferroelectricity, conductivity, and magnetism hinder the discovery of conducting multiferroics. Here, a new path toward metallic multiferroics is provided by theoretically demonstrating the possible compatible nature of ferroelectric distortion, free carriers, and magnetism in electron‐doped PbTiO3 using the hybrid Hartree–Fock density functional theories. Doping with electrons is found to induce metallic conductivity that coexists with and even enhances the ferroelectric distortion in PbTiO3, due to the unique lone‐pair ferroelectricity in this material. The injected excess electrons, in spin‐polarized states, interact with one another in the plane perpendicular to the polar direction, resulting in layer‐arranged ferromagnetism and multiferroics with nonlinear magnetoelectric coupling. These results indicate a means of circumventing conventional restrictions, leading to new types of multifunctional materials in which unusual multiferroic and conductive characteristics are simultaneously present.

Orbital order switching in molecular calculations using GGA functionals: Qualitative errors in materials modeling for electrochemical power sources and how to fix them

We report a qualitative difference in molecular band structures and frontier orbital nodal structures in DFT calculations using GGA vs. hybrid functionals and Hartree Fock in molecules used in electrochemical power sources. This can have a significant effect in applications sensitive to redox potentials and to orbital overlaps (excitations, electron transfer rates) but for which the use of hybrid functionals is impractical, such as solids or interfaces used in electrochemical energy conversion and storage technologies. We show that correct band structures and nodal structures (ordering) of frontier orbitals can be obtained by applying a Hubbard correction to selected atomic states.

Synthesis, crystal structure, and photocatalytical properties of Ba3Ta5O14N

Light yellow Ba3TaV5O14N was successfully synthesized as phase-pure material crystallizing isostructurally to well-known mixed-valence Ba3TaV4TaIVO15. The electronic structure of Ba3Ta5O14N was studied theoretically with a hybrid Hartree–Fock-DFT method. The most stable structure was obtained when nitrogen atoms were placed at 4h sites having fourfold coordination. By incorporating nitrogen, the band gap decreases from ∼3.8eV commonly known for barium tantalum(V) oxides to 2.8eV for the oxide nitride, giving rise to an absorption band well in the visible-light region. Ba3Ta5O14N was also tested for photocatalytic hydrogen formation.

Structure and Properties of Polyfluoride Fn(-) Clusters (n = 3-29).

The electronic and geometrical structures of the neutral Fn and singly negatively charged Fn(-) polyfluorides (n = 3-29) are studied using three levels of theory: density functional theory (DFT) with generalized gradient approximation, hybrid Hartree-Fock-DFT, and hybrid HF-DFT with long-range corrections. For n > 4, each polyfluoride possesses a number of states with different geometries that are closely spaced in total energy. The geometrical structures of the lowest total energy states follow different patterns for the even-n and odd-n Fn(-) anion branches with a preference for higher symmetry geometries. The largest F29(-) anion considered is found to possess Oh symmetry. All the anions beginning with F3(-) are found to possess adiabatic and vertical electron detachment energies exceeding the electron affinities of halogen atoms and are therefore superhalogen anions. Electron affinities, energies of formation, and binding energies show oscillatory behavior as functions of the number n of fluorine atoms. The neutral Fn species are found to be barely stable and are bound by polarization forces. The Fn(-) anions, on the contrary, are quite stable toward the loss of F, F(-), and F2(-), but not to the loss of F2.

A comparison between ab initio calculated and measured Raman spectrum of triclinic albite (NaAlSi3O8)

Albite is one of the most common minerals in the Earth's crust, and its polymorphs can be found in rocks with different cooling histories. The characteristic spectrum of vibration of the albite mineral reflects its structural Si/Al ordering. In this study, we report on the comparison between the Raman spectra measured on a natural and fully ordered (as deduced on the basis of single‐crystal X‐ray diffraction data) ‘low albite’, NaAlSi3O8, and those calculated at the hybrid Hartree–Fock/density functional theory level by employing the WC1LYP Hamiltonian, which has proven to give excellent agreement between calculated and experimentally measured vibrational wavenumbers in silicate minerals. All the 39 expected Ag modes are identified in the Raman spectra, and their wavenumbers and intensities, in different scattering configurations, correspond well to the calculated ones. The average absolute discrepancy is ~3.4 cm−1, being the maximum discrepancy |Δv|max ~ 10.3 cm−1. The very good quality of the WC1LYP results allows for reliable assignments of the Raman features to specific patterns of atomic vibrational motion. Copyright © 2015 John Wiley &amp; Sons, Ltd.

MPW1PW91‐Calculated Structures, IR Spectra, and Frontier Orbitals of Benzoylmethoxythiacalix[4]arene Complexed with Alkali Metal Ions

The molecular structures of four conformers [cone (CONE), partial cone (PACO), 1,2‐alternate (12A), and 1,3‐alternate (13A)] of benzoylmethoxythiacalix[4]arene 1 and the corresponding alkali metal ion complexes were optimized by using a mPW1PW91/6‐31G(d,p) (hybrid Hartree–Fock density functional) calculation method. The 13A conformer was the most stable among the various conformers of 1 because of less steric hindrance. The total electronic and Gibbs free energies, complexation energies, and the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) frontier orbitals of the complexes of 1 with the sodium and potassium ions were analyzed. Among the ditopic complexes, the 13A‐type 1•2Na+ complex exhibited the strongest binding efficiency. The binding efficiency of the CONE‐type 1•Na+ complex was much stronger than that of the 12A‐type 1•Na+ complex. However, the 1•K+ complexes were relatively insensitive to the conformation of host 1. The calculated binding preference of the various conformers is consistent with the empirical solution data. The infrared spectra of the conformers of 1 and their exo complexes with the alkali metal ions were calculated by mPW1PW91 and analyzed.

First-principles study of nitrogen doping and oxygen vacancy in cubic PbTiO3

The structural and electrical properties of the intrinsic PbTiO3, N-doped PbTiO3 and N-VO co-doping PbTiO3 have been studied by using the first-principles calculations based on the hybrid Hartree–Fock density functionals. In addition, Hall effect measurement experiment is performed, both the experimental and theoretical results are compared. In N-doped PbTiO3 system, the N impurity states occur at the top of valance band, and Fermi energy level intersects with valance bands. The band gap becomes narrowed, the N-doped PbTiO3 shows a typical p-type semiconductor characteristic. When oxygen vacancy and N impurity coexist in PbTiO3, the conduction bands move downward and the acceptors are found to be fully compensated.

Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers.

Mo- and W-dichalcogenide compounds have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Here we discover that mechanical deformations provide a route to switching thermodynamic stability between a semiconducting and a metallic crystal structure in these monolayer materials. Based on state-of-the-art density functional and hybrid Hartree-Fock/density functional calculations including vibrational energy corrections, we discover that MoTe2 is an excellent candidate phase change material. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe2 under uniaxial conditions at room temperature. The potential for mechanical phase transitions is predicted for all six studied compounds.

Hybrid Hartree–Fock-density functional theory study of V2O5 three phases: Comparison of bulk and layer stability, electron and phonon properties

We have performed first-principles calculations of the atomic and electronic structures and stability of three (α, β and γ) layered V2O5 phases and their free layers within the same computational approach. In computations, we have used a hybrid exchange–correlation functional within the density functional theory and a basis set of localized atomic orbitals. All the lattice parameters and the atomic positions have been totally optimized and the phonon frequencies at the Γ-point of the Brillouin zone have been obtained. The calculated relative stability of considered bulk phases decreases as α>γ>β. The calculated relative stability of layers differs from that of bulk phases and decreases as β>α⩾γ. The possibility of the folding of V2O5 layers into nanotubes is discussed.

Analysis of the Configurations of a Crystal Surface. Pyrope (Mg3Al2Si3O12) as a Case Study

This paper presents a working methodology to determine all the possible surface configurations of a hkl crystal face. As a case study, we have applied our strategy to individuate the surface configurations of the {100}, {110}, and {112} forms of pyrope (Mg3Al2Si3O12). To identify the most stable surface termination, we have performed an accurate ab initio study of the surface structures and energies at 0 K of the pyrope {100} configurations, by using the hybrid Hartree–Fock/density functional B3LYP Hamiltonian and a localized all-electron Gaussian-type basis set. Even if we are mainly interested in obtaining informations about the habit and genesis of natural garnets included in diamond (and pyrope is the main component of garnets trapped in diamonds), our results could be extremely useful to any researcher involved in the study of the surfaces of whatever crystalline phase.