Exchange-correlation Potential Research Articles

Density functional theory and Monte Carlo simulation insights into electronic structure and magnetic properties in HoSi and CeSi

The electronic and magnetic properties of heavy fermion bulk CeSi and HoSi were studied using density functional theory (DFT) and Monte Carlo simulation (MCS). The exchange correlation (XC) potential was solved by GGA (generalized gradient approximation), TB-mBJ (Tran Blaha modified Becke Johnson) approximations, and also by TB-mBJ and GGA combined with U (Hubbard parameter). Calculations of the electronic structure indicate that CeSi and HoSi compounds are metals and reveal antiferromagnetic behavior. However, the calculations within the TB-mBJ+U approach predict ferrimagnetic behavior for HoSi. The magnetic moments of Ce and Ho in HoSi and CeSi have been estimated. The values of N é el temperature, TN, are found to be 6.61 K and 22.5 K for CeSi and HoSi, respectively. The theoretical TN value of CeSi is very close to that found experimentally, 6.1 K. The maximum of susceptibility is investigated for both compounds.

First principles study of stacking periodicity effects on structural, electronic and magnetic properties of [formula omitted] superlattice

In this article, it is proven that the stacking periodicity has an impact on the structural, electrical, and also magnetic properties of the (MnAs)n/(AlAs)n Superlattice using the full potential linearized augmented plane waves (FP-LAPW) method, which is built on the theory of density functional (DFT). The exchange-correlation potential was determined using the GGA-PBE approximation under ambient conditions. Among these effects, the lattice constant and total magnetic moment of the SLs rise linearly as the number of layers increases. The (MnAs)1/(AlAs)1 SL has a half-metallic behavior; however, the (MnAs)2/(AlAs)2 and (MnAs)3/(AlAs)3 has a semi-metallic behavior, and the origin of the (DOS) and the magnetic moment of the SLs are the d states of the Mn atom. These results demonstrate that controlling the number of MnAs layers may be used to engineer the magnetic properties of the MnAs/AlAs superlattice, leading to a novel spintronic device.

First-principles calculations to investigate structural, mechanical, electronic, optical, and thermoelectric properties of novel cubic double Perovskites X2AgBiBr6 (X=Li, Na, K, Rb, Cs) for optoelectronic devices

ABSTRACT Structural, elastic, electronic, optical, and thermoelectric properties of cubic double perovskites X2AgBiBr6 (X = Li, Na, K, Rb, Cs) were investigated using the density functional theory (DFT) method. The DFT calculations were carried out with various exchange-correlation potentials, e.g. LDA, GGA-PBE, GGA-WC, and hybrid functionals (YS-PBE0). Structural and elastic properties of X2AgBiBr6 demonstrate that these compounds are ionically bonded, elastically stable, ductile, and anisotropic. Calculations show that the compounds are semiconductors with indirect bandgap at the (X- L) point, with bandgap values of 2.124, 2.222, 2.198, 2.209, and 1.902 eV for X2AgBiBr6 (X = Li, Na, K, Rb, and Cs), respectively. Due to their distinguishing optical characteristics and indirect wide bandgap, these compounds might be utilised as absorber layers in solar cells and other optoelectronic devices. Moreover, thermoelectric properties show that the Figure of Merit (ZT) has values of 0.713, 0.723, 0.721, 0.726, and 0.728 for X2AgBiBr6 (X = Li, Na, K, Rb, Cs). The Figure of Merit shows a plateau in the temperature range of 500–900 K, which corresponds to the highest value of ZT. All investigated compounds have holes as the majority of charge carriers. Thermoelectric properties of X2AgBiBr6 compounds reveal that these compounds can be employed in thermoelectric devices.

Density functional descriptions of interfacial electronic structure

Heterogeneous interfaces are central to many energy-related applications in the nanoscale. From the first-principles electronic structure perspective, one of the outstanding problems is accurately and efficiently calculating how the frontier quasiparticle levels of one component are aligned in energy with those of another at the interface, i.e., the so-called interfacial band alignment or level alignment. The alignment or the energy offset of these frontier levels is phenomenologically associated with the charge-transfer barrier across the interface and therefore dictates the interfacial dynamics. Although many-body perturbation theory provides a formally rigorous framework for computing the interfacial quasiparticle electronic structure, it is often associated with a high computational cost and is limited by its perturbative nature. It is, therefore, of great interest to develop practical alternatives, preferably based on density functional theory (DFT), which is known for its balance between efficiency and accuracy. However, conventional developments of density functionals largely focus on total energies and thermodynamic properties, and the design of functionals aiming for interfacial electronic structure is only emerging recently. This Review is dedicated to a self-contained narrative of the interfacial electronic structure problem and the efforts of the DFT community in tackling it. Since interfaces are closely related to surfaces, we first discuss the key physics behind the surface and interface electronic structure, namely, the image potential and the gap renormalization. This is followed by a review of early examinations of the surface exchange-correlation hole and the exchange-correlation potential, which are central quantities in DFT. Finally, we survey two modern endeavors in functional development that focus on the interfacial electronic structure, namely, the dielectric-dependent hybrids and local hybrids.

Thermoelectric properties of Mg-doped mercury selenide HgSe

By using the density functional theory (DFT) in combination with Boltzmann transport theory, the influence of Mg concentrations (x) doping on the thermoelectric properties of Hg[Formula: see text]MgxSe ternary alloys was systematically investigated. The generalized gradient approximations of Perdew–Burke–Ernzerhof (GGA-PBE) have been used to illustrate the exchange correlation potential. The thermodynamic stability of the studied compounds was analyzed in terms of formation energy. Various thermoelectric transport parameters, such as the Seebeck coefficient (S), the thermal conductivity over relaxation time (k/[Formula: see text]), the electrical conductivity over relaxation time ([Formula: see text]/[Formula: see text]), the power factor (PF) and the figure of merit (ZT) have been deduced and discussed. The obtained results of thermoelectric properties show that the studied materials can be useful for thermoelectric devices at room temperature. It can also be seen that Mg concentrations can increase the thermal efficiency of the HgSe alloy.

Implementation of the meta-GGA exchange-correlation functional in numerical atomic orbital basis: With systematic testing on SCAN, rSCAN, and r2SCAN functionals.

Kohn-Sham density functional theory (DFT) is nowadays widely used for electronic structure theory simulations, and the accuracy and efficiency of DFT rely on approximations of the exchange-correlation functional. By including the kinetic energy density τ, the meta-generalized-gradient approximation (meta-GGA) family of functionals achieves better accuracy and flexibility while retaining the efficiency of semi-local functionals. For example, the strongly constrained and appropriately normed (SCAN) meta-GGA functional has been proven to yield accurate results for solid and molecular systems. We implement meta-GGA functionals with both numerical atomic orbitals and plane wave bases in the ABACUS package. Apart from the exchange-correlation potential, we also discuss the evaluation of force and stress. To validate our implementation, we perform finite-difference tests and convergence tests with the SCAN, rSCAN, and r2SCAN meta-GGA functionals. We further test water hexamers, weakly interacting molecules from the S22 dataset, as well as 13 semiconductors using the three functionals. The results show satisfactory agreement with previous calculations and available experimental values.

Machine-learning Kohn–Sham potential from dynamics in time-dependent Kohn–Sham systems

The construction of a better exchange-correlation potential in time-dependent density functional theory (TDDFT) can improve the accuracy of TDDFT calculations and provide more accurate predictions of the properties of many-electron systems. Here, we propose a machine learning method to develop the energy functional and the Kohn–Sham potential of a time-dependent Kohn–Sham (TDKS) system is proposed. The method is based on the dynamics of the Kohn–Sham system and does not require any data on the exact Kohn–Sham potential for training the model. We demonstrate the results of our method with a 1D harmonic oscillator example and a 1D two-electron example. We show that the machine-learned Kohn–Sham potential matches the exact Kohn–Sham potential in the absence of memory effect. Our method can still capture the dynamics of the Kohn–Sham system in the presence of memory effects. The machine learning method developed in this article provides insight into making better approximations of the energy functional and the Kohn–Sham potential in the TDKS system.

Exploring the half-metallic ferromagnetism, dynamical and mechanical stability, optoelectronic and thermoelectric properties of K2NaMI6 (M = Mn, Co, Ni) for spintronic applications

The structural stability, optoelectronic and magnetic characteristics of K2NaMI6 (M = Mn, Co, and Ni) halide double perovskites have been demonstrated to be explained using density functional theory computations. The prominent generalized gradient approximation and integration of the mBJ potential are implemented to estimate the exchange–correlation potential, which is the only unidentified parameter in the state-of-the-art formulism. The structural optimization, mechanical stability criteria, and tolerance factor demonstrate the reliability of the double perovskites in a cubic structure with Fm3m symmetry. The elastic constants facilitated mechanical stability and revealed the brittle nature of these double perovskites. The spin-polarized electronic band profile and the behaviour of the dielectric constant and absorption coefficient in the spin-up and down channels show the presence of half-metallic nature in these materials. Additionally, we examined magnetism and the genesis of the half-metallic gap in this article. The half-metallic and magnetic properties are attributed to the unpaired electrons in the split d-orbitals of the M-sited elements in the crystal field. The Mn-, Co-, and Ni-based double perovskites were found to possess total magnetic moments of 4 μB, 4 μB, and 1 μB, respectively, with the transition metal atoms comprising up the majority of this magnetic moment. The Fermi level’s perfect spin polarisation promotes the potential application of double perovskites in spintronic technology.

Structural, Mechanical, Elastic, Electronic, Magnetic and Optical Properties of Spinel Compounds ATi2S4 (A=Ca, Sr and Ba): AB Initio Study

Abstract This research explored the crystal structure, mechanical and elastic, electronic, magnetic, and optical properties of spinel compoundsATi2S4 (A=Ca, Sr and Ba). The ferromagnetic phase of the compounds was found to be stable, and FP-LAPW method within density functional theory (DFT) was used to calculate and compare the lattice constant a, bulk modulus B0, and its pressure derivative B'. The exchange-correlation potential was also treated with both the GGA and TB-mBJ potential approximation. The results of the study showed that the examined spinels exhibit half-metallic behavior, which makes them a potential material for use in spintronic and optoelectronic devices.

Exchange correlation potentials from full configuration interaction in a Slater orbital basis.

Ryabinkin-Kohut-Staroverov (RKS) theory builds a bridge between wave function theory and density functional theory by using quantities from the former to produce accurate exchange-correlation potentials needed by the latter. In this work, the RKS method is developed and tested alongside Slater atomic orbital basis functions for the first time. To evaluate this approach, full configuration interaction computations in the Slater orbital basis are employed to give quality input to RKS, allowing full correlation to be present along with correct nuclei cusps and asymptotic decay of the wavefunction. SlaterRKS is shown to be an efficient algorithm to arrive at exchange-correlation potentials without unphysical artifacts in moderately-sized basis sets. Furthermore, enforcement of the nuclear cusp conditions will be shown to be vital for the success of the Slater-basis RKS method. Examples of weakly and strongly correlated molecular systems will demonstrate the main features of SlaterRKS.

Dissecting the ingredients of optimally tuned range-separated hybrid models for reliable description of non-adiabatic couplings.

Perusing the non-radiative processes requires a reliable prediction of non-adiabatic couplings (NACs) describing the interaction of two Born-Oppenheimer surfaces. In this regard, the development of appropriate and affordable theoretical methods that accurately account for the NAC terms between different excited-states is desirable. In this work, we develop and validate several variants of the optimally tuned range-separated hybrid functionals (OT-RSHs) for investigating NACs and related properties, such as excited states energy gaps and NAC forces, within the time-dependent density functional theory framework. Particular attention is paid to the influence of the underlying density functional approximations (DFAs), the short- and long-range Hartree-Fock (HF) exchange contributions, and the range-separation parameter. Considering several radical cations and sodium-doped ammonia clusters with the available reference data for the NACs and related quantities as the working models, we have evaluated the applicability and accountability of the proposed OT-RSHs. The obtained results unveil that any combination of the ingredients in the proposed models is not proper for describing the NACs, but a particular compromise among the involved parameters is needed to achieve reliable accuracy. Scrutinizing the results of our developed methods, the OT-RSHs based on the PBEPW91, BPW91, and PBE exchange and correlation DFAs, including about 30% HF exchange at the short-range regime, appeared to be the best performers. We also find that the newly developed OT-RSHs with correct asymptotic exchange-correlation potential have superior performances as compared to their standard counterparts with the default parameters and many earlier hybrids with both fixed and interelectronic distance-dependent HF exchange. The recommended OT-RSHs in this study can hopefully be applicable as computationally efficient alternatives to the expensive wave function-based methods for the systems prone to non-adiabatic properties as well as to screen out the novel candidates prior to their challenging synthesis.

Study of physical properties of the new inorganic perovskites LiSnX3 (X=Br or I): A DFT approach

This work aims to study the optoelectronic and thermoelectric properties of the new perovskites LiSnX3 (X[Formula: see text]Br or I). This study is carried out by using the density functional theory (DFT) combined with the Boltzmann transport theory. First, the investigation of the structural, electronic and optical properties of such materials, using the Generalized Gradient Approximation (GGA), has been performed. This latter method has been performed under the Perdew–Burke–Ernzerhof (GGA–PBE) approximation applying the exchange correlation potential. It is found that the two compounds exhibit a p-type semiconductor characteristic and interesting optical properties. Moreover, the thermoelectric properties of the studied materials, such as the Seebeck coefficient, electrical conductivity, thermal conductivity, figure of merit and power factor, have been examined. It is found that the Seebeck coefficient takes positive values. This finding reveals and confirms the p-type semiconductor characteristic for two studied compounds. On the other hand, such materials display large values of the figure of merit (ZT) in comparison with the existing thermoelectric materials. Such large values of ZT are found for both small or large temperature values. The obtained results of the optoelectronic and thermoelectric properties show that these materials could be useful both for photovoltaic and thermoelectric applications.

Specific features of electromagnetic waves in degenerate two stream electron-ion plasma: self-induced magnetic field

Abstract Generation and amplification of magnetic fields is a subject of interest to both laboratory and astrophysical plasmas. The counter-streaming instability or Weibel instability is a mechanism responsible for self-generating magnetic fields in plasmas. In this paper, we investigate that the non-stationary ponderomotive force of a large amplitude electromagnetic wave (EMWs) propagating through dense two stream quantum electron ion plasma, may lead to the generation of d.c. magnetic fields. It is shown that degeneracy parameters, specifically the exchange correlation potential and Fermi pressure can reduce/control the self-induced magnetic field. Next the linear propagation of EMWs through plasma under consideration has also been considered in the presence of exchange correlation potential along with other quantum effects. Linear dispersion relation, the instability conditions and growth rate have been derived. It is found that the degeneracy parameters stabilize the propagation of instability and lead to reduction of the exponentially growing magnetic field. Threshold scale length of EMWs is also obtained for the stable propagation. The present result may be account helpful to understand the seed magnetic fields in dense plasmas.

Theoretical investigation of Ca0.75Cd0.25X (X = S, Se), a novel ternary alloy, in light of its structural, elastic, electronic, and optical properties

In the current work, using the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the Wien2k code and framed within density functional theory (DFT), we present a theoretical analysis of the structural, elastic, electronic, and optical properties of binary compounds CaX (X = S, Se), and ternary alloys Ca0.75Cd0.25X (X = S, Se). We employ the Wu-Cohen approximation to estimate the exchange-correlation potential. This approximation enables us to accurately determine the elastic and structural properties of the studied materials. Furthermore, we enhance the accuracy of our electronic structure calculations by incorporating the modified Becke Johnson (mBJ) potential. Our calculations for the structural, elastic, electronic and optical parameters of the binary compounds are compared with previously reported studies and are found to be consistent with the available data in the literature. We observe that our materials are semiconductors characterized by an indirect band gap energy (Γ-X) for CaX and (M-Γ) for Ca0.75Cd0.25X. Furthermore, the outcomes of our computations demonstrate that our materials exhibit brittleness, as determined by evaluating the elastic constants and corresponding elastic moduli. To our knowledge, the ternary alloys are investigated for the first time and the obtained results are predictions and still need to undergo experimental validation.

Optoelectronic and thermoelectric properties of Sb2S3 under hydrostatic pressure for energy conversion

This study reports the optoelectronic and thermoelectric properties of antimony trisulfide (Sb2S3) under a hydrostatic pressure of up to 20.4 GPa. The properties were computed based on the full-potential linearized augmented plane wave using the generalized gradient approximation by Perdew, Burke, and Ernzerhof as the exchange-correlation potentials. It was shown that increasing the pressure from 0.00 to 20.4 GPa decreases the calculated bandgap from 1.44 to 0.84 eV. There was a discontinuity in the pressure range of 4.82–6.3 GPa due to the isostructural electronic phase transition. The applied pressure increases the inner electrical polarization. At high pressure, the energy of the negative value of ε1 becomes large, and ε1 itself always remains negative. We observed that the high absorption of Sb2S3 also increases with pressure and the plasmon energy shifts to high energy. The applied pressure increases the static dielectric constant and static refractive index. It was found that the Seebeck coefficients increase with increasing temperature and decrease with increasing pressure. The bipolar effect occurs at low doping levels and high pressure. The optical and thermoelectric properties of Sb2S3 obtained under pressure show that it is suitable for clean energy conversion and optoelectronic applications.

Ab initio study of structural, electronic and magnetic properties of the new full Heusler alloys Co2ZrZ (Z = Pb, Bi, As)

In this paper, the calculations of the structural, electronic, and magnetic properties of new full Heusler Co2ZrZ (Z = Pb, Bi, As) alloys were investigated within the framework of the density functional theory (DFT). Calculations were performed using the Quantum-Espresso package and ultrasoft pseudopotential, based on generalized-gradient approximation (GGA) as the exchange–correlation potential. Using ab initio calculations, properties such as equilibrium lattice constants (a0), bulk modulus (B), electronic band structures, majority and minority of density of states (DOS), as well as the total and atomic magnetic moments of these new compounds were calculated. The results show that the most stable structure of these compounds is the Cu2MnAl phase with a L21 crystal structure. The calculation of the electronic band structure and the majority and minority of DOS show that the Co2ZrPb compound has half-metallic nature with a band gap of 0.33eV. However, based on the band structure and total and partial DOS, the Co2ZrBi and Co2ZrAs compounds exhibit ferromagnetic metallic nature. In addition, the calculation of the magnetic properties indicates that the Co2ZrPb has a total magnetic moment of 2µB, which follows the Slater-Pauling (SP) rule.

Accurate Kohn-Sham auxiliary system from the ground-state density of solids

The Kohn-Sham (KS) system is an auxiliary system whose effective potential is unknown in most cases. It is in principle determined by the ground-state density and it has been found numerically for some low-dimensional systems by inverting the KS equations starting from a given accurate density. For solids, only approximate results are available. In this work, we determine accurate exchange-correlation (xc) potentials for Si and NaCl using the ground-state densities obtained from auxiliary field quantum Monte Carlo calculations. We show that these xc potentials can be rationalized as an ensemble of a few local functions of the density, whose form depends on the specific environment and can be well characterized by the gradient of the density and the local kinetic energy density. The KS band structure can be obtained with high accuracy. The true KS band gap turns out to be larger than the prediction of the local density approximation, but significantly smaller than the measurable photoemission gap, which confirms previous estimates. Finally, our findings show that the conjecture that very different xc potentials can lead to very similar densities and other KS observables is true also in solids, which questions the meaning of details of the potentials and, at the same time, confirms the stability of the KS system.

Physical properties of elastically and thermodynamically stable magnetic AcXO3 (X = Cr, Fe) perovskite oxides: a DFT investigation

Spin-polarized calculations of mechanical, electronic structure, phonon, optical and magnetic properties of AcXO3 (X = Cr, Fe) perovskite oxides (POs) has been computed using the full-potential linearized augmented plane wave method. The modified Becke Johnson (mBJ) approximation has been utilized for exchange-correlation potential and implemented in the WIEN2k code. The negative values of formation energy and the positive fRequencies of the phonon modes show the stability of studied perovskite oxides. The mechanical stability is confirmed through the elastic parameters such as shear modulus (G), Bulk modulus (B), Poisson ratio (ν) and Cauchy pressure. The semiconductor nature with an indirect bandgap is observed for both compounds in both spin channels. The computed electron density contour plot describes the bonding nature of both compounds. The magnetic moments are calculated, which show the major involvement of Fe and Cr atoms in the overall magnetism of studied compounds. The optical response is also evaluated, showing the maximum absorption in the ultraviolet region. The overall analysis of the calculated properties shows that the studied oxide perovskites are suitable for spintronic and optoelectronic applications.

First-principles calculations to investigate structural, electronic, and optical properties of zinc-blende CdxMg(1-x)TeyS(1- y) matched to CdX (X= S, Te) for Predicting critical thickness and corrective Term's effect

We report in the present paper ab initio calculations of the structural, electronic and optical properties of zinc blende quaternary alloys CdxMg(1-x)TeyS(1-y) lattice matched to CdX (X = S, Te) exploring the density functional method within the generalized gradient approximation (GGA). From the obtained values of the ground state energy, we have determined the structural parameters and the band gap energies. We found an underestimation of the gap energies obtained for both (GGA-PBE) and the Modified Becke-Johnson exchange potential (TB-mbJ) approximations for the CdS, CdTe, MgS, and MgTe. Even though mbJ significantly corrects the energy gaps, it is no longer valid to properly reproduce the exchange-correlation potential. Consequently, we tend to use the corrective term ΔE to exploit it to enhance the electronic band structures.Moreover, we have reported the concentration depending on the refractive index and the dielectric function using (mbJ). The critical thickness, for which the lattice of the CdxMg(1-x)TeyS(1-y) matches CdS and CdTe, respectively, are determined. The results show that for all Cd and Te incorporated amounts, the CdxMg(1-x)TeyS(1-y) lattice matched to CdX (X = S, Te) substrate alloys exhibits a semiconducting behavior with a small lattice mismatching ratio which might make CdxMg(1-x)TeyS(1-y)/CdX materials promising and useful for optoelectronic applications.

Elastic, electronic, optical and thermoelectric properties of Ca5Si2N6 and Sr5Ge2N6 ternary nitrides

In order to expand the scope of application of ternary nitrides and explore more thoroughly their potential as novel materials, ab initio calculations founded on density functional theory are executed to examine the mechanical, electronic, optical and transport properties of Ca5Si2N6 and Sr5Ge2N6 nitrides. These compounds are thermodynamically stable established on coherent energy and enthalpy of formation, with Ca5Si2N6 having the best stability. The estimated values of elastic constants and their derived properties suggest that the items under examination are mechanically stable, ductile and elastically anisotropic. Electronic properties have been evaluated using the GGA and TB-mBJ exchange-correlation potentials. The band structure calculated via TB-mBJ reveals that these nitrides are semiconductors, where Ca5Si2N6 has a direct energy gap of 3.55 eV and Sr5Ge2N6 an indirect band-gap of 3.15 eV. By combining Debye temperature and band-gap values, it was concluded that the nitridosilicate compound can be used as phosphor material. According to the optical response, which was examined in terms of dielectric function, complex refractive index, absorption, reflectivity and energy damage function, the potential applications of Ca5Si2N6 and Sr5Ge2N6 have been discussed. Further, thermoelectric properties are computed utilizing the semi-classical Boltzmann transport notion. The obtained results show a large strength operator and depressed thermal conductivity driving to rising figure of merit values, particularly at 300 K (0.978–0.986), signifying that the studied ternary nitrides are favorable nominees for thermoelectric employments at both low and room temperatures.