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Register a new userBeyond the quasiparticle approximation: Fully self-consistent $GW$ calculations
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We present quasiparticle (QP) energies from fully self-consistent $GW$ (sc$GW$) calculations for a set of prototypical semiconductors and insulators within the framework of the projector-augmented wave methodology. To obtain converged results, both finite basis-set corrections and $k$-point corrections are included, and a simple procedure is suggested to deal with the singularity of the Coulomb kernel in the long-wavelength limit, the so called head correction. It is shown that the inclusion of the head corrections in the sc$GW$ calculations is critical to obtain accurate QP energies with a reasonable $k$-point set. We first validate our implementation by presenting detailed results for the selected case of diamond, and then we discuss the converged QP energies, in particular the band gaps, for a set of gapped compounds and compare them to single-shot $G_0W_0$, QP self-consistent $GW$, and previously available sc$GW$ results as well as experimental results.
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We present an approach to calculate the electronic structure for a range of materials using the quasiparticle self-consistent GW method with vertex corrections included in the screened Coulomb interaction W. This is achieved by solving the Bethe-Salpeter equation for the polarization matrix at all k-points in the Brillouin zone. We refer to this method as QSGW^. We show that including ladder diagrams in W can greatly reduce the band gap overestimation of RPA-based QSGW. The resultant discrepency of the calculated band gap in this method is then attributed mostly to the fact that electron-phonon contributions to W are neglected; which would allow one to then obtain an estimate for the size of this effect. We present results for a range of systems from simple sp semiconductors to the strongly correlated systems NiO and CoO. Results for systems where the RPA-based QSGW band gap is larger than expected are investigated, and an estimate for the Frolich contribution to the gap is included in a few polar compounds where QSGW can overestimate the gap by as much as 2 eV. The improvement over QSGW for the dielectric constants is also presented
Finding an accurate ab initio approach for calculating the electronic properties of transition metal oxides has been a problem for several decades. In this paper, we investigate the electronic structure of the transition metal monoxides MnO, CoO, and NiO in their undistorted rock-salt structure within a fully iterated quasiparticle self-consistent GW (QPscGW) scheme. We study the convergence of the QPscGW method, i.e., how the quasiparticle energy eigenvalues and wavefunctions converge as a function of the QPscGW iterations, and we compare the converged outputs obtained from different starting wavefunctions. We find that the convergence is slow and that a one-shot G$_0$W$_0$ calculation does not significantly improve the initial eigenvalues and states. It is important to notice that in some cases the path to convergence may go through energy band reordering which cannot be captured by the simple initial unperturbed Hamiltonian. When we reach a fully iterated solution, the converged density of states, band-gaps and magnetic moments of these oxides are found to be only weakly dependent on the choice of the starting wavefunctions and in reasonably good agreement with the experiment. Finally, this approach provides a clear picture of the interplay between the various orbitals near the Fermi level of these simple transition metal monoxides. The results of these accurate {it ab initio} calculations can provide input for models aiming at describing the low energy physics in these materials.
Within the framework of the full potential projector-augmented wave methodology, we present a promising low-scaling $GW$ implementation. It allows for quasiparticle calculations with a scaling that is cubic in the system size and linear in the number of $k$ points used to sample the Brillouin zone. This is achieved by calculating the polarizability and self-energy in the real space and imaginary time domain. The transformation from the imaginary time to the frequency domain is done by an efficient discrete Fourier transformation with only a few nonuniform grid points. Fast Fourier transformations are used to go from real space to reciprocal space and vice versa. The analytic continuation from the imaginary to the real frequency axis is performed by exploiting Thieles reciprocal difference approach. Finally, the method is applied successfully to predict the quasiparticle energies and spectral functions of typical semiconductors (Si, GaAs, SiC, and ZnO), insulators (C, BN, MgO, and LiF), and metals (Cu and SrVO$_3$). The results are compared with conventional $GW$ calculations. Good agreement is achieved, highlighting the strength of the present method.
We present spin wave dispersions in MnO, NiO, and $alpha$-MnAs based on the quasiparticle self-consistent $GW$ method (qsgw), which determines an optimum quasiparticle picture. For MnO and NiO, qsgw results are in rather good agreement with experiments, in contrast to the LDA and LDA+U description. For $alpha$-MnAs, we find a collinear ferromagnetic ground state in qsgw, while this phase is unstable in the LDA.
We present an approach to calculate the optical absorption spectra that combines the quasiparticle self-consistent GW method [Phys. Rev. B, 76 165106 (2007)] for the electronic structure with the solution of the ladder approximation to the Bethe-Salpeter equation for the macroscopic dielectric function. The solution of the Bethe-Salpeter equation has been implemented within an all-electron framework, using a linear muffin-tin orbital basis set, with the contribution from the non-local self-energy to the transition dipole moments (in the optical limit) evaluated explicitly. This approach addresses those systems whose electronic structure is poorly described within the standard perturbative GW approaches with as a starting point density-functional theory calculations. The merits of this approach have been exemplified by calculating optical absorption spectra of a strongly correlated transition metal oxide, NiO, and a narrow gap semiconductor, Ge. In both cases, the calculated spectrum is in good agreement with the experiment. It is also shown that for systems whose electronic structure is well-described within the standard perturbative GW, such as Si, LiF and h-BN, the performance of the present approach is in general comparable to the standard GW plus Bethe-Salpeter equation. It is argued that both vertex corrections to the electronic screening and the electron-phonon interaction are responsible for the observed systematic overestimation of the fundamental bandgap and spectrum onset.
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