Self-consistent GW Approach Research Articles

Broken-symmetry self-consistent GW approach: Degree of spin contamination and evaluation of effective exchange couplings in solid antiferromagnets

We adopt a broken-symmetry strategy for evaluating effective magnetic constants J within the fully self-consistent GW method. To understand the degree of spin contamination present in broken-symmetry periodic solutions, we propose several extensive quantities demonstrating that the unrestricted self-consistent GW preserves the broken-symmetry character of the unrestricted Hartree-Fock solutions. The extracted J are close to the ones obtained from multireference wave-function calculations. In this paper, we establish a robust computational procedure for finding magnetic coupling constants from self-consistent GW calculations and apply it to solid antiferromagnetic nickel and manganese oxides.

Probing ionization potential, electron affinity and self-energy effect on the spectral shape and exciton binding energy of quantum liquid water with self-consistent many-body perturbation theory and the Bethe–Salpeter equation

An accurate theoretical prediction of ionization potential (IP) and electron affinity (EA) is key in understanding complex photochemical processes in aqueous environments. There have been numerous efforts in literature to accurately predict IP and EA of liquid water, however with often conflicting results depending on the level of theory and the underlying water structures. In a recent study based on hybrid-non-self-consistent many-body perturbation theory (MBPT) Gaiduk et al (2018 Nat. Commun. 9 247) predicted an IP of 10.2 eV and EA of 0.2 eV, resulting in an electronic band gap (i.e. electronic gap (IP-EA) as measured by photoelectron spectroscopy) of about 10 eV, redefining the widely cited experimental gap of 8.7 eV in literature. In the present work, we show that GW self-consistency and an implicit vertex correction in MBPT considerably affect recently reported EA values by Gaiduk et al (2018 Nat. Commun. 9 247) by about 1 eV. Furthermore, the choice of pseudo-potential is critical for an accurate determination of the absolute band positions. Consequently, the self-consistent GW approach with an implicit vertex correction based on projector augmented wave (PAW) method on top of quantum water structures predicts an IP of 10.2, an EA of 1.1, a fundamental gap of 9.1 eV and an exciton binding (Eb) energy of 0.9 eV for the first absorption band of liquid water via the Bethe–Salpeter equation (BSE). Only within such a self-consistent approach a simultanously accurate prediction of IP, EA, Eg, Eb is possible.

Two-photon absorption spectrum of liquid water and the effect of nondiagonal self-energy elements in the self-consistent GW approach on the band gap

Two-photon absorption spectrum of liquid water and the effect of nondiagonal self-energy elements in the self-consistent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>G</mml:mi><mml:mi>W</mml:mi></mml:mrow></mml:math> approach on the band gap

Ground state properties of 3d metals from self-consistent GW approach

The self consistent GW approach (scGW) has been applied to calculate the ground state properties (equilibrium Wigner–Seitz radius SWZ and bulk modulus B) of 3d transition metals Sc, Ti, V, Fe, Co, Ni, and Cu. The approach systematically underestimates SWZ with average relative deviation from the experimental data of about 1% and it overestimates the calculated bulk modulus with relative error of about 25%. It is shown that scGW is superior in accuracy as compared to the local density approximation but it is less accurate than the generalized gradient approach for the materials studied. If compared to the random phase approximation, scGW is slightly less accurate, but its error for 3d metals looks more systematic. The systematic nature of the deviation from the experimental data suggests that the next order of the perturbation theory should allow one to reduce the error.

Structure-property relationships in cubic cuprous iodide: A novel view on stability, chemical bonding, and electronic properties.

Based on the combination of density functional theory and theory-group methods, we performed systematic modeling of γ-CuI structural design at the atomistic level. Being started from the metallic copper lattice, we treated a crystal assembly as a stepwise iodination process characterized in terms of a sequence of intermediate lattice geometries. These geometries were selected and validated via screening of possible structural transformations. The genesis of chemical bonding was studied for three structural transformations by analyzing the relevant changes in the topology of valence electron densities. We determined structural trends driven by metal-ligand coupling. This allowed us to suggest the improved scenario of chemical bonding in γ-CuI. In particular, the unconventional effect of spatial separation of metallic and covalent interactions was found to be very important with respect to the preferred arrangements of valence electrons in the iodination process. We rigorously showed that useful electronic and optical properties of γ-CuI originate from the combination of two separated bonding patterns-strong covalency established in I-Cu tetrahedral connections and noncovalent interactions of copper cores is caused by the 3d10 closed-shell electron configurations. The other finding of ours is that the self-consistency of the GW calculations is crucial for correctly determining the dynamic electronic correlations in γ-CuI. Detail reinvestigation of the quasi-particle energy structure by means of the self-consistent GW approach allowed us to explain how p-type electrical conductivity can be engineered in the material.

Understanding the optical properties of ZnO1−xSx and ZnO1−xSex alloys

ZnO1−xYx with chalcogen element Y exhibits intriguing optoelectronic properties as the alloying strongly impacts the band-gap energy Eg(x). In this work, we analyze and compare the electronic structures and the dielectric responses of Zn(O,S) and Zn(O,Se) alloys by means of the density functional theory and the partially self-consistent GW approach. We model the crystalline stability from the total energies, and the results indicate that Zn(O,S) is more stable as alloy than Zn(O,Se). We demonstrate also that ion relaxation strongly affects total energies, and that the band-gap bowing depends primarily on local relaxation of the bonds. Moreover, we show that the composition dependent band-gap needs to be analyzed by the band anti-crossing model for small alloying concentration, while the alloying band-bowing model is accurate for strong alloying. We find that the Se-based alloys have a stronger change in the band-gap energy (for instance, ΔEg(0.50) = Eg(ZnO) – Eg(x = 0.50) ≈ 2.2 eV) compared with that of the S-based alloy (ΔEg(0.50) = 1.2 eV), mainly due to a stronger relaxation of the Zn–anion bonds that affects the electronic structure near the band edges. The optical properties of the alloys are discussed in terms of the complex dielectric function ε(ω) = ε1(ω) + iε2(ω) and the absorption coefficient α(ω). While the large band-gap bowing directly impacts the low-energy absorption spectra, the high-frequency dielectric constant ε∞ is correlated to the intensity of the dielectric response at energies above 4 eV. Therefore, the dielectric constant is only weakly affected by the non-linear band-gap variation. Despite strong structural relaxation, the high absorption coefficients of the alloys demonstrate that the alloys have well-behaved optoelectronic properties.

A self-consistent GW approach to the van der Waals potential for a helium dimer.

van der Waals interaction between two helium (He) atoms is studied by calculating the total energy as a function of the He-He distance within the self-consistent GW approximation, which is expected to behave correctly in the long wavelength limit. In the Born-Oppenheimer (BO) approximation, the pair potential curve has its minimum value at 2.87 Å, which is somewhat larger than the local density approximation result, 2.40 Å, and is closer to previous quantum chemistry results. The expectation value for the interatomic distance, calculated by solving the Schrödinger equation for the two nuclei problem using the BO potential energy curve, is 30 Å, which is smaller but of the same order as previous experimental and theoretical results.

QSGW+DMFT: an electronic structure scheme for the iron pnictides and beyond

While in strongly correlated materials one often focuses on local electronic correlations, the influence of non-local exchange and correlation effects beyond band-theory can be pertinent in systems with more extended orbitals. Thus in many compounds an adequate theoretical description requires the joint treatment of local and non-local self-energies. Here, I will argue that this is the case for the iron pnictide and chalcogenide superconductors. As an approach to tackle their electronic structure, I will detail the implementation of the recently proposed scheme that combines the quasi-particle self-consistent GW approach with dynamical mean-field theory: QSGW+DMFT. I will showcase the possibilities of QSGW+DMFT with an application on BaFe2As2. Further, I will discuss the empirical finding that in pnictides dynamical and non-local correlation effects separate within the quasi-particle band-width.

Linearized self-consistent GW approach satisfying the Ward identity

We propose a linearized self-consistent GW approach satisfying the Ward identity. The vertex function derived from the Ward-Takahashi identity in the limit of $\mathbit{q}=0$ and $\ensuremath{\omega}\ensuremath{-}{\ensuremath{\omega}}^{\ensuremath{'}}=0$ is included in the self-energy and the polarization function as a consequence of the linearization of the quasiparticle equation. Due to the energy dependence of the self-energy, the Hamiltonian is a non-Hermitian operator and quasiparticle states are nonorthonormal and linearly dependent. However, the linearized quasiparticle states recover orthonormality and fulfill the completeness condition. This approach is very efficient, and the resulting quasiparticle energies are greatly improved compared to the nonlinearized self-consistent GW approach, although its computational cost is not much increased. We show the results for atoms and dimers of Li and Na compared with other approaches. We also propose convenient ways to calculate the Luttinger-Ward functional $\ensuremath{\Phi}$ based on a plasmon-pole model and calculate the total energy for the ground state. As a result, we conclude that the linearization improves overall behaviors in the self-consistent GW approach.

Band structures of Cu2ZnSnS4 and Cu2ZnSnSe4 from many-body methods

We calculate the band structures of kesterite and stannite Cu2ZnSnS4 and Cu2ZnSnSe4, using a state-of-the-art self-consistent GW approach. Our accurate quasiparticle states allow to discuss: the dependence of the gap on the anion displacement; the key-role of the nonlocality of the exchange-correlation potential to obtain good structural parameters; the reliability of less expensive hybrid functional and generalized gradient approximation+U approaches. In particular, we show that even if the band gap is correctly reproduced by hybrid functionals, the band-edge corrections are in disagreement with self-consistent GW results, which have decisive implications for the positioning of the defect levels in the band gap.