Local Density Functional Theory Research Articles

Aromaticity of Heterocyclic Compounds and Their Corrosion Inhibition Property: Experimental and Theoretical Analysis.

Heterocycle derived moieties, namely, N-(4-methoxyphenyl)-1-(1H-pyrrol-2-yl)methanimine (MPM), 1-(furan-2-yl)-N-(4-methoxyphenyl)methanimine (FMM), and N-(4-methoxyphenyl)-1-(thiophen-2-yl)methanimine (MTM), were synthesized followed by analysis of their structural aspects using FTIR and 1H NMR spectroscopic techniques. The corrosion retarding abilities of the same were distinguished by gravimetric and certain electrochemical measures for mild steel in 0.5 M H2SO4, and MTM was obtained with maximum inhibition efficiency of 97.93% at 250 mg L-1 concentration; the thermodynamic and activation parameters were recorded in this regard. The results were further seen to be supported by various surface studies: SEM-EDS, XPS, AFM, contact angle, and UV-visible spectroscopy. Potentiodynamic polarization studies unveiled the mixed nature of heterocyclic inhibitors with overriding anodic effect. Furthermore, the adsorption of inhibitors over mild steel coupons demarcates the prevalence of physical and chemical interactions in the environment. In addition, the computational studies, global and local reactivity, molecular dynamics, and density functional theory, were employed and the experimental results obtained were found in correlation with the theoretical results.

Pseudo-Jahn-Teller Effect Breaks the pH Dependence in Two-Electron Oxygen Electroreduction.

The hydrogenation of small molecules (like O2 and CO2) often exhibits strong activity dependence on pHs because of discrepant proton donor environments. However, some catalysts can show seldom dependence on two-electron oxygen electroreduction, a sustainable route of O2 hydrogenation to hydrogen peroxide (H2O2). In this work, a pH-resistant oxygen electroreduction system arising from the pseudo-Jahn-Teller effect is demonstrated. Thorough operando Raman spectra, local environment analyses and density function theory simulations, the lattice distortion of TiOxFy that introduces the pseudo-Jahn-Teller effect contributing to regulating local pHs at electrode-electrolyte interfaces and the absorption/desorption of key *OOH intermediate is revealed. Consequently, as comparison to 78.6% activity attenuation for common catalyst, the TiOxFy displays minor activity decay (3.2%) in the pH range of 1-13 with remarkable Faradaic efficiencies (93.4-96.4%) and H2O2 yield rates (595-614mgcm-2h-1) in the current densities of 100-1000mAcm-2. Further techno-economics analyses display the H2O2 production cost dependent on pHs, giving the lowest H2O2 price of $0.37kg-1. The present finding is expected to provide an additional dimension to pseudo-Jahn-Teller effect that leverages systems beyond traditional conception.

Study on the minimum miscibility pressure and phase behavior of CO2–shale oil in nanopores

In recent years, CO2 enhanced oil recovery has gained traction in shale oil reservoirs. Shale oil reservoirs are characterized by numerous nanopores, where shale oil exhibits unique phase behavior due to nano–confinement effects. Traditional theoretical model and microfluidic experiment often fail to accurately depict the minimum miscible pressure (MMP) of CO2–shale oil in nanopores. In this study, nanofluidic experiments were conducted to investigate the MMP of CO2–alkanes in single pore channels and porous media channels with depths of 30 nm and 100 nm. Additionally, a novel thermodynamic model of gas–liquid–adsorbed phase equilibrium, which considers the capillary force and the shift in the critical property, was developed by integrating simplified local density functional theory. The feasibility of the method was determined by comparing the measured MMP with the MMP obtained using the newly established thermodynamic model, resulting in errors of 10.804 % and 5.545 %. Finally, phase behavior and MMP for two typical oil samples in block A in China were conducted using the newly established thermodynamic model. The results revealed that as the pore radius decreases, the area of the two–phase region enclosed by the phase envelope contracts progressively. In addition, the saturation pressure decreases gradually for shale oil with a higher content of light components as the molar fraction of the injected CO2 increases, whereas for shale oil with higher heavy component content, it increases as the CO2 molar fraction increases. The MMP of CO2–shale oil in 10 nm pores is 70.188 bar at 392.75 K of sample 1 and 83.007 bar at 374.72 K of sample 2. Compared to the bulk phase, the MMP was reduced by 63.76 % and 57.64 %.

Magnetic ground states and hyperfine interactions in YMnO[formula omitted] using density functional theory

This study employs the Local Density functional theory (DFT) and the DFT+U extension, to better describe the localized d-Mn states, and therefore analyze six magnetic configurations of the Mn ions in hexagonal manganite YMnO3. The respective study was complemented through hyperfine interaction calculations of different atomic configurations. Our focus is on six distinct magnetic configurations, complemented by thorough Hyperfine Interactions (HFI) calculations targeting the most plausible configuration. We find the P6’3 and P63 configurations are the most probable, aligning with previous research in the field. These configurations also show a strong correlation with experimental observations of magnetic moments, providing new insights into the magnetic ordering of YMnO3. Our results highlight the intricate relationship between magnetic moments and the lattice structure of YMnO3, offering valuable insights into the material’s magnetic properties.

Splay-induced order in systems of hard tapers.

The main objective of this work is to clarify the role that taper-shaped elongated molecules, i.e., molecules with one end wider than the other, can play in stabilizing orientational order. The focus is exclusively on entropy-driven self-organization induced by purely excluded volume interactions. Drawing an analogy to RM734 (4-[(4-nitrophenoxy)carbonyl]phenyl-2,4-dimethoxybenzoate), which is known to stabilize ferroelectric nematic (N_{F}) and nematic splay (N_{S}) phases, and assuming that molecular biaxiality is of secondary importance, we consider monodisperse systems composed of hard molecules. Each molecule is modeled using six collinear tangent spheres with linearly decreasing diameters. Through hard-particle, constant-pressure Monte Carlo simulations, we study the emergent phases as functions of the ratio between the smallest and largest diameters of the spheres (denoted as d) and the packing fraction (η). To analyze global and local molecular orderings, we examine molecular configurations in terms of nematic, smectic, and hexatic order parameters. Additionally, we investigate the radial pair distribution function, polarization correlation function, and the histogram of angles between molecular axes. The last characteristic is utilized to quantify local splay. The findings reveal that splay-induced deformations drive unusual long-range orientational order at relatively high packing fractions (η>0.5), corresponding to crystalline phases. When η<0.5, only short-range order is affected, and in addition to the isotropic liquid, only the standard nematic and smectic-A liquid crystalline phases are stabilized. However, for η>0.5, apart from the ordinary nonpolar hexagonal crystal, three additional frustrated crystalline polar blue phases with long-range splay modulation are observed: antiferroelectric splay crystal (Cr_{S}P_{A}), antiferroelectric double-splay crystal (Cr_{DS}P_{A}), and ferroelectric double-splay crystal (Cr_{DS}P_{F}). Finally, we employ Onsager-Parsons-Lee local density functional theory to investigate whether any sterically induced (anti)ferroelectric nematic or smectic-A type of ordering is possible for our system, at least in a metastable regime.

Analytical approximation to the local softness and hypersoftness and to their applications as reactivity indicators.

Local density functional theory derivatives of the electron density have been calculated analytically for the set of canonical hydrogenic orbitals; original solutions have been obtained using the novel density gradient theorem. Results for the first and second derivatives of electron density over N (number of electrons) and over μ (chemical potential) have been demonstrated. Calculations of the state functions ΔN, ΔE, and Δμ disturbed by an external potential Δv(r) have been obtained via the concept of alchemical derivatives. The local softness s(r) and local hypersoftness [ds(r)/dN]v have been proved to provide crucial chemical information on the sensitivity of orbital density to the disturbance of the external potential Δv(r), leading to electron exchange ΔN and the corresponding changes of the state functions ΔE, Δμ. The results are fully compatible with the well-understood character of atomic orbitals in chemistry and open a perspective to applications to atoms, free or bonded.

Degradation mechanism of BPA under VUV irradiation: efficiency contribution and DFT calculations.

Bisphenol A (BPA) is regarded as a hazardous pollutant that exists widely in aquatic environments, posing a severe threat to human health. In this study, a vacuum ultraviolet (VUV) lamp emitting a hybrid of 254nm and 185nm light was used to degrade BPA. Results indicated that photolysis via 254nm wavelength accounted for 24.93% for BPA decay, while indirect oxidation was responsible for 52.27% of decay. Results confirmed that the degradation of BPA under VUV illumination mainly occurred via photo-excited degradation and ·OH electrophilic addition reactions based on average local ionization energy (ALIE) calculation and density functional theory (DFT) calculations. Therefore, only light with a wavelength of 254nm was able to induce the first three excited states of BPA, forming the electron transition type of n → π* from O atom to a single benzene ring and π → π* in the single benzene ring. Indirect oxidation by ·OH occurred as it preferentially attacked the C6 atom in BPA ring A. Moreover, the energy required for photo-excited degradation was about twofold than that of ·OH electrophilic addition reactions.

Critical assessment of Co-Cu phase diagram from first-principles calculations

Using first-principles alloy theory, we perform a systematic study of the Co-Cu phase diagram. Calculations are carried out for ferromagnetic and paramagnetic ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ solid solutions with face-centered-cubic (fcc) crystal structure. We find that the equilibrium volumes and magnetic states are crucial for a quantitative description of the thermodynamics of the Co-Cu system at temperatures up to 1400 K. In particular, the paramagnetic state of Cu-rich alloys with persisting local magnetic moments is shown to be responsible for the solubility of a small amount of Co in fcc Cu whereas the excess entropy in the ferromagnetic Co-rich region critically depends on the adopted lattice parameters. None of the common local or semilocal density functional theory approximations have the necessary accuracy for the lattice parameters when compared to the experimental data. The predicted ab initio Co-Cu phase diagram is in good agreement with the measurements and CALPHAD data, making it possible to gain a deep insight into the various contributions to the Gibbs free energy. The present study provides an atomic-level description of the thermodynamic quantities controlling the limited mutual solubility of Co and Cu and highlights the importance of high-temperature magnetism.

Hyperfine interaction of H-divacancy in diamond

We present a first principles density functional theory study of microscopic properties of hydrogen defect centres in diamond. Several configurations, involving interstitial hydrogen impurities, have been considered either forming with other defects, such as hydrogen defects and vacancies. The atomic structures, and hyperfine parameters of hydrogen result compared with the experimental data on electrically active centres in synthetic diamond. Based on Local density functional theory our calculations are in excellent agreement with one interpretation of electron paramagnetic resonance of hydrogen in diamond.

Simple correction to bandgap problems in IV and III–V semiconductors: an improved, local first-principles density functional theory

We report results from a fast, efficient, and first-principles full-potential Nth-order muffin-tin orbital (FP-NMTO) method combined with van Leeuwen–Baerends correction to local density exchange-correlation potential. We show that more complete and compact basis set is critical in improving the electronic and structural properties. We exemplify the self-consistent FP-NMTO calculations on group IV and III–V semiconductors. Notably, predicted bandgaps, lattice constants, and bulk moduli are in good agreement with experiments (e.g. we find for Ge 0.86 eV, 5.57 , 75 GPa versus measured 0.74 eV, 5.66 , 77.2 GPa). We also showcase its application to the electronic properties of 2-dimensional h-BN and h-SiC, again finding good agreement with experiments.

Dopant-related electron trap states in Lu2O3:Ta

Electronic structure of Ta-doped cubic (bixbyite-type) lutetium oxide was analyzed using augmented plane wave with local orbitals density functional theory (DFT) calculations with meta-generalized gradient approximation (meta-GGA, mGGA) Räsänen, Pittalis and Proetto functional. Two doping sites, three oxidation states of the dopant and optional interstitial oxygen (Oi) were considered. The calculations indicate that introduction of the dopant results in energy levels below conduction band, with strong contribution of Ta 5d orbitals. The position of the levels depends strongly on the oxidation state of the dopant. Depending on the composition, two distinct electron traps with depths of 1.5–1.2 eV and 0.9–0.2 eV were revealed. Introduction of Oi at proximity of the Ta dopant results in partial overlap of the Ta-related defect states with conduction band, which makes the traps very shallow, deteriorating the electron trapping properties. In some cases, shallow hole traps can also be attributed to the Oi. The calculated traps structure is in good accord with the experimental data.

Interaction between Bi Dopants and Intrinsic Defects in LiNbO3 from Local and Hybrid Density Functional Theory Calculations.

The interactions between Bi dopants including Bi-substituting Li (BiLi) and Bi-substituting Nb (BiNb) and the intrinsic antisite defects (NbLi) and Li vacancies (VLi) in LiNbO3 are investigated using local and hybrid density functional theories. Three charge-compensated defect clusters, BiLi4+ + NbLi4+ + 8VLi-, BiLi4+ + 4VLi-, and BiLi4+ + BiNb0 + 4VLi-, are modeled in this work to investigate the effects of the Bi concentration. The most stable cluster configurations, the Bi-doping stability in the clusters, and the electronic state interaction between Bi and intrinsic defects have been studied in detail. It is found that BiLi4+ has a stronger electron-capturing ability than NbLi4+ in Bi-doped congruent LiNbO3. The BiLi-doping-induced local lattice distortion and the electron-trapping behavior remain unchanged with increasing Bi-doping concentration. However, the position of the Bi defect states in the band gap is found to be shifted in congruent LiNbO3. This is mainly attributed to the large lattice relaxation induced by the large number of Li vacancies instead of the ionic level redistribution caused by the direct interaction between Bi and intrinsic defects.

Effects of calcium on planar fault energies in ternary magnesium alloys

We are motivated by the need to design magnesium alloys that are free of rare-earth additions, but, nevertheless, forgeable and free of strong basal texture. It has become recently clear that calcium is a promising candidate to replace the economically strategic rare-earth elements. To this end, we focus on the planar faults that typically bound partial glide dislocations of the hcp lattice. We have made first-principles calculations to examine the generalized stacking fault energy (SFE) of the basal, first- and second-order pyramidal planes. We examine the changes in fault energy and anisotropy for increasing alloy concentrations and the effect of low concentrations of calcium. For the calculation of the SFEs from first principles, we use the non-self-consistent Harris-Foulkes approximation to the local density functional theory. We demonstrate that while this approximation leads to high computational efficiency, there is no significant loss of precision compared to the self-consistent Hohenberg-Kohn functional. We go beyond all previous work in which the alloying element is assumed to reside within the fault and instead address the more realistic situation in which the SFE is modified by the remote presence of the impurity. This allows us to determine whether segregation is expected and we find that while elements do segregate to the basal fault they do not to pyramidal faults. Nevertheless, in either case, the fault energy is strongly modified by alloying. This argues that either a long-ranged electronic structure effect is in play, or the fault energy modification is affected by the atomic size difference---particularly large in the case of Ca. We find that Mg-Li-Ca and Mg-Zn-Ca alloys show a remarkable decrease in anisotropy, which is consistent with their known high strength and formability. In favorable cases, this comes about by strengthening basal slip rather than weakening nonbasal slip. The Ca contribution increases inversely with the atomic size of the alloying element, allowing us to speculate that alloying effects are generally atomic size effects.

BaTiO3 polar surface in ultrahigh vacuum calculated by local density functional theory with a large supercell

Whether the intrinsic screening of depolarization by electrons and holes exists is essential for the fundamental ferroelectric properties such as domains and size effect. Despite many studies of domain boundary conduction, only clean BaTiO3 polar surfaces in ultrahigh vacuum (UHV) exhibit ohmic conductance at low field, clear polarization dependence and consistent temperature dependence, which prove the existence of electron and hole layer induced by depolarization field. Therefore, we study TiO2 polar surface (c+, c− surface) of BaTiO3 local density functional theory (LDA) with a large supercell consisting of 11 layers of TiO2 and10 layers of BaO. These calculations confirm the electron and hole layer formation at the surface shown by experiments in ultrahigh vacuum and Ginzburg-Landau theory and resolve some inconsistencies between them.

Electronic band structure and optical absorption of CdSe doped with interstitial oxygen molecules

We have investigated the electronic structure and optical properties of CdSe doped with molecular oxygen by using first-principle local density-functional theory (DFT) based on a hybrid functional. We find that interstitial oxygen molecule introduces two kinds of states in CdSe: filled states which are degenerate and strongly hybridized with the top of the valence bands and one empty state lying in the middle of the gap. The origin of these states can be traced to the 1πg molecular orbitals of O2. We compared the optical absorption spectra of CdSe with and without molecular oxygen. States introduced by oxygen molecules lower the absorption onset of CdSe to ∼1.0eV while hybridization causes redistribution of the oscillator strengths of CdSe transitions from the ultraviolet to the infrared. Our results demonstrate that doping small molecules into semiconductors can alter the electronic and optical properties of the host in ways different from atomic dopants.

Effects of Enhanced Flexibility and Pore Size Distribution on Adsorption-Induced Deformation of Mesoporous Materials.

Here, we present a new model of adsorption-induced deformation of mesoporous solids. The model is based on a simplified version of local density functional theory in the framework of solvation free energy. Instead of density, which is treated as constant here, we used film thickness and pore radius as order parameters. This allows us to obtain a self-consistent system of equations describing simultaneously the processes of gas adsorption and adsorbent deformation, as well as conditions for capillary condensation and evaporation. In the limit of infinitely rigid pore walls, when the film becomes several monolayers thick, the model reduces to the well-known Derjaguin-Broekhoff-de Boer theory for pores with cylindrical geometry. We have investigated the effects of enhanced flexibility of the solid as well as the influence of pore size distribution on the adsorption/deformation process. The formulation of the theory allows to determine the average pore size and its width from the desorption branch of the strain isotherm only. The model reproduces the nonmonotonic behavior of the strain isotherm at low relative pressure. Furthermore, we discuss the effect of rigidity of the adsorbent on the pore size distribution, showing qualitatively different results of the adsorption isotherms for rigid and highly flexible materials, in particular, the shift of evaporation pressure to lower values and the absence of a limiting value of the loading at high relative pressure. We also discuss the results of the theory with respect to experimental data obtained from the literature.

Effect of Hartree-Fock pseudopotentials on local density functional theory calculations.

Density functional theory (DFT) can run into serious difficulties with localized states in elements such as transition metals with occupied d states and oxygen. In contrast, including a fraction of the Hartree-Fock exchange can be a better approach for such localized states. Here, we develop Hartree-Fock pseudopotentials to be used alongside DFT for solids. The computational cost is on a par with standard DFT. Calculations for a range of II-VI, III-V and group-IV semiconductors with diverse physical properties show an observably improved band gap for systems containing d-electrons, pointing to a new direction in electronic theory.

Implications of the DFT+U method on polaron properties in energy materials

To model polaronic behavior in strongly correlated transition-metal oxides with ab initio methods, one typically requires a level of theory beyond that of local density or general gradient density functional theory (DFT) approximations to account for the strongly correlated $d$-shell interactions of transition-metal oxides. In the present work, we utilize density functional theory with additional on-site Hubbard corrections $(\mathrm{DFT}+U)$ to calculate polaronic properties in two lithium ion battery cathode materials, ${\mathrm{Li}}_{x}{\mathrm{FePO}}_{4}$ and ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$, and two photocatalytic materials, ${\mathrm{TiO}}_{2}$ and ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. We investigate the effects of the $+U$ on-site projection on polaronic properties. Through systematic comparison with hybrid functional calculations, it is shown that $+U$ projection in these model materials can impact upon the band gap, polaronic hopping barrier, and polaronic eigenstate offset from the band edges in a nontrivial manner. These properties are shown to have varying degrees of coupling and dependence on the $+U$ projection in each example material studied, which has important implications for arriving at systematic material predictions of polaronic properties in transition-metal oxides.

Size dependent electronic properties of silicon quantum dots—An analysis with hybrid, screened hybrid and local density functional theory

We use an efficient projection scheme for the Fock operator to analyze the size dependence of silicon quantum dots (QDs) electronic properties. We compare the behavior of hybrid, screened hybrid and local density functionals as a function of the dot size up to ∼800 silicon atoms and volume of up to ∼20 nm3. This allows comparing the calculations of hybrid and screened hybrid functionals to experimental results over a wide range of QD sizes. We demonstrate the size dependent behavior of the band gap, density of states, ionization potential and HOMO level shift after ionization. We also demonstrate how the use of Graphical Processing Units (GPUs) can further accelerate such calculations.

Photoionization of open-shell halogen atoms endohedrally confined in C60

SynopsisDipole photoionization of atomically open-shell halogen-endofullerenes Cl,Br,I@C60 are calculated using time-dependent local density functional theory and a spherical potential model. Hybridization of the valence np orbital of the atom with a C60 p orbital is found to strongly modify the ionization parameters.