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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., Comment: 14 Latex pages, 10 Figures with multiple subfigures

Solar cells based on conventional semiconductors have low efficiency in converting solar energy into electricity because the excess energy beyond the gap of an incident solar photon is converted into heat by phonons. Here we show by ab initio methods that the presence of strong Coulomb interactions in strongly correlated insulators (SCI) causes the highly photo-excited electron-hole pair to decay fast into multiple electron-hole pairs via impact ionization (II). We show that the II rate in the insulating $M_1$ phase of vanadium dioxide (chosen for this study as it is considered a prototypical SCI) is two orders of magnitude higher than in Si and much higher than the rate of hot electron/hole decay due to phonons. Our results indicate that a rather broad class of materials may be harnessed for an efficient solar-to-electrical energy conversion that has been not considered before., Comment: 5 Latex pages, 3 figures, and also supplementary material

During the last decade, ab initio methods to calculate electronic structure of materials based on hybrid functionals are increasingly becoming widely popular. In this Letter, we show that, in the case of small gap transition metal oxides, such as VO2, with rather subtle physics in the vicinity of the Fermi-surface, such hybrid functional schemes without the inclusion of "expensive" fully self-consistent GW corrections fail to yield this physics and incorrectly describe the features of the wave function of states near the Fermi-surface. While a fully self-consistent GW on top of hybrid functional approach does correct these wave functions as expected, and is found to be in general agreement with the results of a fully self-consistent GW approach based on semilocal functionals, it is much more computationally demanding as compared to the latter approach for the benefit of essentially the same results., Comment: 5 two-column pages, 2 figures

We study the optical response of VO2 in the M1 insulating phase using methods based on density functional theory in its most recent developments. We start from a hybrid functional approach which may be a good starting point to carry out many-body perturbation theory since, as we show, it gives a qualitatively and to some degree quantitatively correct description of the insulating phase. In order to calculate the dielectric function, first, using hybrid density functional wavefunctions we have added GW corrections on top of the hybrid density functional calculations and we solved the so-called Bethe Salpeter equation to include effects of correlations of the electron-hole pair created upon photon absorption. We find that the effects of electron correlations are very important and they show up as a strong contribution of the electron-hole interaction in calculating the effects of the Bethe-Salpeter equation in the electron/hole pair excitation on top of the interacting group state., Comment: 12 double-column pages, 11 postscript figures