Exchange-correlation Functionals Research Articles

A Theoretical Benchmark of the Geometric and Optical Properties for 3d Transition Metal Nanoclusters via Density Functional Theory.

Understanding structure-property relationships in atomically precise metal nanoclusters is vital in finding selective and tunable catalysts. In this study, density functional theory (DFT) was used to benchmark seven exchange correlation functionals at different basis sets for 17 atomically precise nanoclusters against experimentally determined geometries, band gaps, and optical gaps. The set contains both monometallic and bimetallic clusters that possess at least two types of 3d transition metals (specifically, Cu, Ni, Fe, or Co). The benchmark highlights that PBE0 is a good functional to use regardless of the basis set, and Minnesota functionals do well with respect to specific metals. Further, while long-range corrected functionals overestimate band and optical gaps, they model absorption features better than the other considered functionals. The study additionally looks at the photoinduced hydrogen evolution reaction (HER) and the CO2 reduction mechanism on nanoclusters reported from the literature.

Significant improvement in structural, electronic, optical and thermoelectric properties of PdTe2 in bulk and monolayer phase: A G0W0+BSE approach

Using first principles calculations structural, electronic, thermoelectric and optical properties of PdTe2 in bulk and monolayer were investigated. The calculated lattice parameters with different exchange correlation functionals show that the results obtained with addition of van der Waals corrections (vdW-DFC09x) are very well matched with the results obtained experimental. For correct prediction of electronic band gap, G0W0 approximation is used and the calculated band structure reveal that PdTe2 in bulk form has metallic nature as attained in experimental studies. In addition, the electronic band gap results with G0W0 approximation depicted that PdTe2 in monolayer phase is a semiconductor material with a direct band gap of 1.12 eV. The optical spectra have been calculated via many-body perturbation theory (MBPT) by solving Bethe-Salpeter equation (BSE) (G0W0+BSE). Ultimately, the results of optical spectra demonstrated that PdTe2 in monolayer phase has strong optical absorption within visible light wavelengths than the bulk phase which is good for solar cell applications. Thermoelectric (TE) properties of PdTe2 in bulk and monolayer structures have been studied using BoltzTraP code. The TE properties calculations indicated that better performance can be obtained by reducing the dimensionality of materials. The figure of merit was found to be above unity for monolayer phase. This paper successfully exploited the enhancement of the figure of merit values with the monolayer structure of PdTe2 material.

Exploring the optical property of InAs1−xSbx-based nBn structures for high-temperature mid-wave infrared detection

InAs0.9Sb0.1-based nBn structures for mid-wave infrared detection are designed and prepared by molecular beam epitaxy. The structural, electrical, and optical properties are characterized, based on which the density-functional theory calculation is carried out by combining with the hybrid HSE06 exchange-correlation function. The results demonstrate that (i) the calculated bandgap for InAs0.9Sb0.1 is well consistent with that of optical spectroscopies (∼266 meV, or a cutoff wavelength of ∼4.66 μm), suggesting the practical effectivity of the theoretical model; (ii) the valence band offset of the unbiased InAs0.9Sb0.1/AlAs0.1Sb0.9 nBn structure is determined as ∼105 meV, with ignorable influence on the hole transport of the devices by considering the rather low dark current when working at a high temperature of ∼150 K; and (iii) the conduction band offset of InAs0.9Sb0.1/GaSb is roughly ∼0.6 eV. This work not only provides a strategy for effectively evaluating the InAs1−xSbx bandgap both theoretically and experimentally but also offers a more reliable basis for fabricating high-performance mid-infrared detectors with high operating temperatures.

Study of the relationship between pressure and sensitivity of energetic materials based on first-principles calculation.

In order to study the relationship between the sensitivity and pressure of energetic materials, six kinds of energetic materials were selected as the research object. The crystal structure, electronic, and phonon properties under hydrostatic pressure of 0 ~ 45 GPa were calculated by first principles. The calculation results show that the lattice parameters and band gap values of these six energetic materials decrease with the increase of pressure. The peak of the density of states decreases and moves to the low energy direction, and the electrons become more active. Meanwhile, the effect of pressure on the sensitivity of the energetic materials is analyzed based on the multi-phonon up-pumping theory. The number of doorway modes and integral of projected phonon density of states under high pressure is calculated. The results show that both of them increase with the increase of pressure. And the smaller the value of the band gap, the larger the number of doorway modes and integral of projected phonon density of states, and the more sensitive the energetic material is. All calculations are performed using the Materials Studio software based on density functional theory. The Perdew-Burke-Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) is used to calculate the exchange correlation function, and the Grimme dispersion correction method is used to deal with the weak intermolecular interaction. The structure of the compound was optimized by BFGS algorithm. The linear response is used to calculate the phonon properties of energetic materials. The plane wave cutoff energy was set to 830eV. The K-point grids of TATB, FOX-7, TNX, RDX, TNT, and HMX were chosen as 2 × 2 × 2, 2 × 2 × 1, 2 × 1 × 1, 1 × 1 × 1, 1 × 2 × 1, and 2 × 1 × 2.

Structural, elastic, mechanical, electronic, and optical properties of cubic K2Pb2O3 from first-principle study.

Based on first principles, the structure, elasticity, mechanics, electronics, and optical properties of cubic K2Pb2O3 were studied. The structural parameters calculated by this method are close to the previous theoretical results. The elastic constant, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and mechanical stability are studied, and it is shown that cubic K2Pb2O3 is mechanically stable, isotropic, and brittleness. The electrical conductivity and chemical bonding of cubic K2Pb2O3 were analyzed based on the calculated band structure, density of states (DOS), and bond populations. The dispersion of optical functions, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, and loss function, is displayed and analyzed. All computations have been carried out based on density functional theory (DFT) as implemented in the CASTEP code. The norm conservation pseudopotential method is used to exchange correlation functionals within the generalized gradient approximation (GGA).

Optoelectronic and mechanical properties of gallium arsenide alloys: Based on density functional theory

First principles calculations based on density functional theory (DFT) were performed to investigate the structural, electronic, optical and mechanical properties of pristine GaAs compound and its alloy; Ga0.75Al0.25As, Ga0.75In0.25As, Ga0.75Sn0.25As, Ga0.75Ti0.25As. WIEN2K and Quantum expresso (QE) codes were adopted for calculations using generalized gradient approximation (GGA) in Perdew-Burke Erzenhoff (PBE) as exchange correlation function for both codes. Full potential linear augmented plane wave (FPLAPW) with the local orbital method was adopted as implement in WIEN2K code. In QE code, norm-conserving pseudopotentials were employed on a plane-wave expansion of the wave functions. Structural and electronic properties were elaborated since their result gives information about the optical and mechanical performance. Electronic band structure and optical parameters were performed using WIEN2K code. Underestimation of band gap observed from DFT calculations were corrected by using Modified Becke and Johnson (mBJ). Mechanical components were determined using QE with thermo_pw package. Lattice constant, volume, bulk modulus and other physical parameters were calculated for structural properties. Discrepancy in these parameters as observed in crystal structure is associated to difference in ionic radius of host and substituted atom. The results of band structure and density of states were calculated for electronic properties. All the studied compounds were semiconductors in nature except Ga0.75Sn0.25As which displayed metallic character. Optical parameters including extinction coefficient, absorption coefficient, refractive index, optical conductivity, optical reflectivity and energy loss function have been computed from the dielectric function at energy range of 0 to 25 eV using the Kramers-Kronig transformations. Calculated elastic function were used to compute the mechanical properties such as anisotropic, brittle characteristics, stiffness and many others. All the results were compared with available theoretical and experimental records.

Thermodynamic properties of rhodium—A first principle study

The high-pressure and high-temperature thermodynamic properties of rhodium (up to 2000 K and 300 GPa) are presented using the first principle approach within the quasi-harmonic approximation. The thermal Helmholtz free energy includes the contribution of both phonon vibrations and electronic excitations. The performance of three popular exchange-correlation functionals—local density approximation [Perdew et al., Phys. Rev. B 23, 5048 (1981)], Perdew–Burke–Ernzerhof generalized gradient approximation (PBE) [Perdew et al., Phys. Rev. Lett. 77, 3865 (1996)], PBE modified for dense solids [Perdew et al., Phys. Rev. Lett. 100, 136406 (2008)] are shown. The simulated thermal expansion coefficient, isobaric heat capacity, mode-Grüneisen parameter, thermodynamic average Grüneisen parameter, and bulk modulus are compared with the available experimental and theoretical reports. The contribution of thermal electronic excitations to the obtained thermodynamic parameters is significant at low pressure and high temperatures, except in bulk modulus, where it is small. The pressure-dependent elastic constant coefficient (Cij) and the Debye temperature are computed at 0 K. The Pugh ratio calculated from Cij indicates that rhodium undergoes brittle to ductile transitions at an average pressure of 7.45 GPa.

Modeling of High-Harmonic Generation in the C60 Fullerene Using Ab Initio, DFT-Based, and Semiempirical Methods.

We report calculations of the high-harmonic generation spectra of the C60 fullerene molecule carried out by employing a diverse set of real-time time-dependent quantum chemical methods. All methodologies involve expanding the propagated electronic wave function in bases consisting of the ground and singly excited time-independent eigenstates obtained through the solution of the corresponding linear-response equations. We identify the correlation and exchange effect in the spectra by comparing the results from methods relying on the Hartree-Fock reference determinant with those obtained using approaches based on the density functional theory with different exchange-correlation functionals. The effect of the full random-phase approximation treatment of the excited electronic states is also analyzed and compared with the configuration interaction singles and the Tamm-Dancoff approximation. We also showcase the fact that the real-time extension of the semiempirical method INDO/S can be effectively applied for an approximate description of laser-driven dynamics in large systems.

Simple hole transporting material based 2,7- carbazole for perovskite solar cells: Structural, photophysical, and theoretical studies

A simple carbazole-based hole transport material, termed HTM1, has been deposited as thin films. The samples were elaborated via a Physical Vapor Deposition to ensure the production of high-quality thin films. The study of the morphological and photophysical properties of the investigated compound was revealed through Scanning Electron Microscopy, Atomic Force Microscopy, Ultraviolet–Visible, and Photoluminescence spectroscopy. A flat surface with low roughness values was observed in the SEM and AFM images. Moreover, the obtained films revealed a higher absorption in the UV region. Optical parameters such as the energy gap (2.9 eV) and refractive index were determined using transmittance measurements. The temperature-dependent photoluminescence over a range of 77–300 K has also been investigated. It is found that, as the temperature decreases, a new emission band appears, which may be due to the formation of excimers. Added to that, theoretical investigation of the highest occupied molecular orbital, the lowest unoccupied molecular orbital, the density of states, and the optical energy gap were studied using density functional theory (DFT) with the hybrid B3LYP exchange-correlation function and the 6-311G (d,p) basis set. Absorption spectra was generated using the TD-DFT method. The absorption maximum obtained (398 nm) for HTM1 closely matched with the experimental value (393 nm). Following these characterizations, simulation studies on the photovoltaic performances of the HTM1 for perovskite solar cells have been done. This cell provided a maximum PCE of 21.31%.

Automated all-functionals infrared and Raman spectra

AbstractInfrared and Raman spectroscopies are ubiquitous techniques employed in many experimental laboratories, thanks to their fast and non-destructive nature able to capture materials’ features as spectroscopic fingerprints. Nevertheless, these measurements frequently need theoretical and computational support in order to unambiguously decipher and assign complex spectra. Linear-response theory provides an effective way to obtain the higher-order derivatives needed, but its applicability to modern exchange-correlation functionals and pseudopotential formalism remains limited. Here, we devise an automated, open-source, user-friendly approach based on density-functional theory and the electric-enthalpy functional to allow seamless calculation from first principles of infrared absorption and reflectivity, together with zone-center phonons, static dielectric tensor, and Raman spectra. By employing a finite-displacement and finite-field approach, we allow for the use of any functional, as well as an efficient treatment of large low-symmetry structures. Additionally, we propose a simple scheme for efficiently sampling the Brillouin zone at different electric fields. To demonstrate the capabilities of the present approach, we study ferroelectric LiNbO3 crystal as a paradigmatic example, and predict infrared and Raman spectra using various (semi)local, Hubbard corrected, and hybrid functionals. Our results also show how PBE0 and extended Hubbard functionals (PBEsol+U+V) yield for this case the best match in term of peak positions and intensities, respectively.

Quantum-Chemistry Study of the Hydrolysis Reaction Profile in Borate Networks: A Benchmark.

This investigation involved an ab initio and Density Functional Theory (DFT) analysis of the hydrolysis mechanism and energetics in a borate network. The focus was on understanding how water molecules interact with and disrupt the borate network, an area where the experimental data are scarce and unreliable. The modeled system consisted of two boron atoms, bridging oxygen atoms, and varying numbers of water molecules. This setup allows for an exploration of hydrolysis under different environmental conditions, including the presence of OH- or H+ ions to simulate basic or acidic environments, respectively. Our investigation utilized both ab initio calculations at the MP2 and CCSD(T) levels and DFT with a range of exchange-correlation functionals. The findings indicate that the borate network is significantly more susceptible to hydrolysis in a basic environment, with respect to an acidic or to a neutral pH setting. The inclusion of explicit water molecules in the calculations can significantly affect the results, depending on the nature of the transition state. In fact, some transition states exhibited closed-ring configurations involving water and the boron-oxygen-boron network; in these cases, there were indeed more water molecules corresponding to lower energy barriers for the reaction, suggesting a crucial role of water in stabilizing the transition states. This study provides valuable insights into the hydrolysis process of borate networks, offering a detailed comparison between different computational approaches. The results demonstrate that the functionals B3LYP, PBE0, and wB97Xd closely approximated the reference MP2 and CCSD(T) calculated reaction pathways, both qualitatively in terms of the mechanism, and quantitatively in terms of the differences in the reaction barriers within the 0.1-0.2 eV interval for the most plausible reaction pathways. In addition, CAM-B3LYP also yielded acceptable results in all cases except for the most complicated pathway. These findings are useful for guiding further computational studies, including those employing machine learning approaches, and experimental investigations requiring accurate reference data for hydrolysis reactions in borate networks.

Superconductivity in transition metal carbides, TaC and HfC: A density functional theory study with local, semilocal, and nonlocal exchange–correlation functionals

We studied electronic, dynamical, and superconducting properties of TaC and HfC using local, semilocal, and nonlocal exchange–correlation functionals in the density functional theory. It turns out that there is a slight shift of Fermi energy and the corresponding density of states with different functionals. However, the phonon frequencies of, especially, the optical modes for both TaC and HfC shift by moderately larger values. The most important differences are observed for the electron–phonon interactions where the electron–phonon coupling constant, λ≈0.9, calculated using the nonlocal correlation functionals, vdW-DF2 and vdW-DF3 for TaC, are in the closest agreement with the experimental results. Although the Kohn anomalies and Fermi surface nesting are critical for the electron–phonon coupling, a significant contribution to λ, for TaC, comes from the regions of the Brillouin zone where the matrix elements have a predominant role. Moreover, the optical modes give 20%–23% of λ for TaC and 50%–57% for HfC.

Strain-induced modification in thermal properties of monolayer 1T-ZrS2 and ZrS2/ZrSe2 heterojunction.

This paper systematically analyzes the phonon dispersion curves of single-layer ZrS2, ZrSe2, and ZrS2/ZrSe2 heterostructures under different strains. The phonon spectra and thermal parameters of the three structures were obtained based on the density functional perturbation theory method. The upper limits of strain that different monolayers and heterojunctions can withstand were studied. The monolayers ZrSe2 and ZrS2 can withstand up to 8% biaxial tensile strain, and the ZrS2/ZrSe2 heterojunction can withstand up to 6% biaxial tensile strain. In addition, the van der Waals force of the heterojunction may cause phonon tearing in the vertical direction. The application of biaxial tensile strain can adjust the thermal properties of the system to a large extent, which is similar to the strain effect in the pristine case. When the temperature rises, the entropy enthalpy of the three structures also gradually increases, the free energy gradually decreases, and the heat capacity of the system gradually increases until it tends to be stable. Taking single-layer ZrS2 as an example, we analyzed the change curve of thermal properties of single-layer ZrS2 under tensile strain. The results show that with the gradual increase of strain, the crystal's entropy, enthalpy, and free energy change differently. In addition, the heat capacity increases slowly under high temperatures. When all systems reach the limit strain, the phonon spectrum appears to have an imaginary frequency, and the thermal properties decrease significantly. This paper uses the first-principle calculation method based on density functional theory, and the PBE exchange-correlation function based on generalized gradient approximation (GGA) is selected for a specific calculation. The density functional perturbation theory (DFPT) calculates the full kinetic matrix. Because the lattice constants of ZrS2 and ZrSe2 are similar and have similar periodicity, the corresponding unit cells are used for structural optimization and property calculation. The Brillouin zone is integrated using the K points generated by the Monkhorst-pack method. For single-layers ZrS2 and ZrSe2, 8 × 8 × 1K-point grid is selected, and for ZrS2/ZrSe2 heterojunction, 8 × 8 × 2K-point grid is selected. A vacuum layer of 30Å was added in the vertical direction to avoid interlayer interaction. The non-conservative pseudopotential method is used to optimize the structure, and the optimization convergence is set as follows: the cutoff energy is set to 700eV, the convergence threshold of the maximum force between atoms is 0.01eV/Å, the convergence threshold of the maximum energy change is set to 1 × 10-9eV, and the convergence threshold of the maximum displacement is 0.001Å.

Theoretical Photoelectron Spectroscopy of Quadruple-Bonded Dimolybdenum(II,II) and Ditungsten(II,II) Paddlewheel Complexes: Performance of Common Density Functional Theory Methods.

We have revisited the gas-phase photoelectron spectra of quadruple-bonded dimolybdenum(II,II) and ditungsten(II,II) paddlewheel complexes with modern density functional theory methods and obtained valuable calibration of four well-known exchange-correlation functionals, namely, BP86, OLYP, B3LYP*, and B3LYP. All four functionals were found to perform comparably, with discrepancies between calculated and experimental ionization potentials ranging from <0.1 to ∼0.5 eV, with the lowest errors observed for the classic pure functional BP86. All four functionals were found to reproduce differences in ionization potentials (IPs) between analogous Mo2 and W2 complexes, as well as large, experimentally observed ligand field effects on the IPs, with near-quantitative accuracy. The calculations help us interpret a number of differences between analogous Mo2 and W2 complexes through the lens of relativistic effects. Thus, relativity results in not only significantly lower IPs for the W2 complexes but also smaller HOMO-LUMO gaps and different triplet states relative to their Mo2 counterparts.

First-principles study of stability of small vacancy clusters in aluminum

The vacancy cluster is one of the most common defects in aluminum alloys. Using density-functional theory-based first-principles calculations, the energetics of small vacancy clusters in aluminum, including voids, stacking fault tetrahedra (SFT), and vacancy platelets on the {111} plane, was investigated. It is found that the significant difference in monovacancy formation energies reported in literatures is mainly related to their used exchange-correlation functions. On average, LDA is the most reliable approximation for the monovacancy formation energies in Al, followed by PBE, PBEsol, PW91, and AM05. The results in this work confirm that the divacancy in Al is indeed energetically unfavorable. Moreover, vacancy clusters with a size smaller than five in any form are unstable against their corresponding number of isolated monovacancies. The SFT is the most stable form of most small vacancy clusters, followed by the void and vacancy platelet. These results are helpful to understand the experimentally observed size distributions of vacancy clusters in Al.

Ab-initio study of point defects in Th and U alloy

Point defect formation energies are calculated along with several structural and elastic properties for the fcc (α) and bcc (β) Th crystal structures. Th-self defects and single uranium impurity defects are modeled in the fcc phase, whereas only Th self-defects are modeled in the high temperature bcc phase. Additionally, an analysis of several well known exchange-correlation functionals is given in regard to defect modeling performance. Overall, defect formation energies in the bcc phase were lower in energy compared to their defect equivalents in the fcc phase. In both phases, vacancies were found be the most stable, with the bcc vacancy being ∼0.8 eV lower in energy than the fcc vacancy, regardless of form of the exchange-functional. The most stable interstitial defects were in the octahedral and <110> dumbbell positions for the fcc and bcc phases, respectively. It was also found that the <110> and <111> dumbbell defects were unstable in the fcc phase, but not in the bcc phase. Overall, the choice of exchange-correlation functional was found to have almost a negligible effect on the calculated formation energies. The most notable difference between functionals was in the prediction of the vacancy defect formation energies, which may be attributed to differences in the way each functional handles exchange and correlation in regions of low electron density relative to the bulk of a material, such as in the internal surface of a vacancy or vacancy cluster.

Structural, phononic, opto-electronic and elasto-mechanical properties of LiXBe (X = N, P) half Heusler compounds

The plethora of properties associated with half Heusler compounds has made them apposite materials for functional applications significantly in thermoelectrics, solar harvesting, semiconducting and spintronics. The current study invested the exertions in unveiling the details of physical properties of LiXBe (X = N, P) compounds using DFT based VASP and WIEN2k codes considering PBE-GGA and TB-mBJ exchange correlation functionals. Titled compounds successfully assured the stability checks regarding energetic, lattice dynamic and mechanical. By means of electronic structure calculations, TB-mBJ functional disclosed the precise band gaps of 6.912 and 2.340 eV for LiNBe and LiPBe compounds respectively. Fat-band character analysis and partial charge density distributions in HOMO and LUMO orbitals revealed the governess of band edges around Fermi-level by homo-site (N-2p & N-2s) states in LiNBe and hetero-site (P-2p & Li-2s) states in LiPBe compound. The COHP examination and bond order characteristics evidenced comparatively strongest bonding interactions of X–Be than Li–Be bonding pair. The calculated opto-electronic properties; complex dielectric function, optical conductivity, loss function and refractive index have been discussed with substantial details. Physio-mechanical parameters and mechanical anisotropy have been comprehensibly elaborated.

A comprehensive investigation on the physical properties of SiC polymorphs for high-temperature applications: A DFT study based on GGA and hybrid HSE06 exchange correlation functionals

This investigation reports the structural, electronic, bonding, elastic, mechanical, thermodynamic, thermal and optical characteristics of SiC polymorphs by DFT based GGA and hybrid HSE06 ab-initio computational methodology. SiC exhibits a multitude of structural orientations, with cubic (3C SiC) and hexagonal (4H SiC and 6H SiC) being the most prevalent. In my research, I aim to gain a comprehensive understanding of these SiC polymorphs and their potential suitability for high-temperature applications, especially as a cladding material for accident-tolerant fuel (ATF) in new-generation nuclear power plants (NPPs). The structural stability is ensured by geometry optimization and the relaxed parameters are agreed well with the previously reported findings. All the studied SiC polytypes reveal the semiconducting electronic property. By utilizing the hybrid HSE06 functional, the obtained electronic band gap meets the experimental results more closely than that obtained by the GGA approximation. The mechanical stability of all studied SiC polymorphs is evident from their elastic and mechanical properties, showcasing a brittle nature. The analysis of bond populations reveals the coexistence of both ionic and covalent bonds within the SiC structure. The thermoelectric and thermal characteristics of mechanically stable SiC polymorphs revealing that, all the compounds possess high lattice thermal conductivity and high melting point. Among these, 3C SiC stands out with the highest thermal conductivity of 172 Wm-1K−1 at 300 K and a melting temperature of 2840 K. The relatively low dielectric constant, high absorption coefficient and broad reflectivity spectra with > 90 % reflection in the UV region suggest that, SiC polymorphs can also be the potential candidates for optoelectronic devices. All the extreme physical stabilities make SiC polymorphs suitable for practical applications in high temperature environments.

Theoretical approach on characterizing structural and mechanical properties of industrial alloys

With the goal of entering Indonesia’s golden dream 2045 in mineral technology and utilizations, expansive researches covering both experimental and theoretical aspects are urgently required. Here, we provide theoretical study on structural and mechanical properties of various nickel-based alloys based on first-principles, density functional theory (DFT) technique. First, we evaluate the accuracy of several exchange-correlation functionals (LDA-PZ, GGA-PBE, GGA-PBEsol) by comparing the calculated lattice constants for each system with the known experimental results. Based on the best functional, we analyze structural changes caused by the mixture of different atom to the principal element of nickel. Finally, we derived the related mechanical properties for all systems based on the calculated elastic constants. Furthermore, the elastic constants can also be used to predict the stability of alloy structure.

Two‐Time Quantum Fluctuations Approach and Its Relation to the Bethe–Salpeter Equation

Correlated quantum many‐particle systems out of equilibrium are of high interest in many fields, including correlated solids, ultracold atoms, or dense plasmas. Accurate theoretical description of these systems is challenging both, conceptionally and with respect to computational resources. A quantum fluctuations approach is recently presented, which is equivalent to the nonequilibrium GW approximation that promises high accuracy at low computational cost. The method exhibits process time scaling that is linear in the number of time steps, like the G1–G2 scheme, however, at a much reduced computer memory cost. In a second publication, this approach is extended to the two‐time exchange–correlation functions and the dynamic density response properties. Herein, the properties of this approach are analyzed in more detail. The physical meaning of the central approximation, the quantum polarization approximation, is established. It is demonstrated that the method is equivalent to the Bethe–Salpeter equation for the two‐time exchange–correlation function when the generalized Kadanoff–Baym ansatz with Hartree–Fock propagators is applied.