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Register a new userAb-initio calculations of Many-Body effects in liquids: the electronic excitations of water
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We present ab-initio calculations of the excited state properties of liquid water in the framework of Many-Body Greens function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. These calculations are made possible by exploiting the insensitivity of screening effects to a particular configuration. The resulting spectra are strongly modified by many-body effects, both concerning peak energies and lineshapes, and are in good agreement with experiments.
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We show, by means of ab-initio calculations, that electron-electron correlations play an important role in potassium-doped picene ($K_x$-picene), recently characterized as a superconductor with $T_c = 18K$. The inclusion of exchange interactions by means of hybrid functionals reproduces the correct gap for the undoped compound and predicts an antiferromagnetic state for $x=3$, where superconductivity has been observed. The latter finding is compatible with a sizable value of the correlation strength, in agreement with simple estimates. Our results highlight the similarity between potassium-doped picene and alkali-doped fulleride superconductors.
Cyclometalled Ir(III) compounds are the preferred choice as organic emitters in Organic Light Emitting Diodes. In practice, the presence of the transition metals surrounded by carefully designed ligands allows the fine tuning of the emission frequency as well as a good efficiency of the device. To support the development of new compounds the experimental measurements are generally compared with ab-initio calculation of the absorption and emission spectra. The standard approach for these calculations is TDDFT with hybrid exchange and correlation functional like the B3LYP. Due to the size of these compounds the application of more complex quantum chemistry approaches can be challenging. In this work we used Many Body Perturbation Theory approaches (in particular the GW approximation with the Bethe-Salpeter equation) implemented in gaussian basis sets, to calculate the quasiparticle properties and the adsorption spectra of six cyclometalled Ir(III) complexes going behind TDDFT. In the presented results we compared standard TDDFT simulation with BSE calculations performed on top on perturbative G 0 W 0 and accounting for eigenvalue self consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrate on Ir(ppy) 3 performing GW-BSE simulations starting from different DFT exchange and correlation potentials.
We present an ab initio $GW$ self-energy calculation of the electronic structure of LaNiO$_2$. With respect to density-functional theory we find that in $GW$ the La 4$f$ states undergo an important $+$2 eV upward shift from the Fermi level, while the O 2$p$ states are pulled down by $-$1.5 eV, thus reinforcing the charge-transfer character of this material. However, $GW$ many-body effects leave the $d$-like bands at the Fermi level almost unaffected, so that the Fermi-surface topology is preserved, unlike in cuprates.
We propose a new, alternative method for ab-initio calculations of the electronic structure of solids, which has been specifically adapted to treat many-body effects in a more rigorous way than many existing ab-initio methods. We start from a standard band-structure calculation for an effective one-particle Hamiltonian approximately describing the material of interest. This yields a suitable set of one-particle basis functions, from which well localized Wannier functions can be constructed using a method proposed by Marzari and Vanderbilt. Within this (minimal) basis of localized Wannier functions the matrix elements of the non-interacting (one-particle) Hamiltonian as well as the Coulomb matrix elements can be calculated. The result is a many-body Hamiltonian in second quantization with parameters determined from first principles calculations for the material of interest. The Hamiltonian is in the form of a multi-band Hamiltonian in second quantization (a kind of extended, multi-band Hubbard model) such that all the standard many-body methods can be applied. We explicitly show how this approach can be solved in the simplest many-body approximation, the mean-field Hartree-Fock approximation (HFA), which takes into account exact exchange and corrects for self-interaction effects.
The first part of this article centers on the fact that key features of the dynamical response of weakly-correlated materials (the alkalis, Al), have been found experimentally to differ qualitatively from simple-model behavior. In the absence of ab initio theory, the surprises embodied in the experimental data were imputed to effects of dynamical correlations. We summarize results of ab initio investigations of linear response, performed within time-dependent density-functional theory (TDDFT), in which the unexpected features of the observed spectra are shown to be due to band-structure effects. Contrary to conventional wisdom, the response cannot be understood universally, in terms of a simple scaling with the density, on going from metal to metal (e.g., through the alkali series) --even the shape of the dispersion curve for the plasmon energy is system-specific. The second part of this article starts out with the observation that a similar ab initio study of systems with more complex electronic structures would require the availability of a realistic approximation for the dynamical many-body kernel entering the density-response function in TDDFT. Thus, we outline a diagrammatic alternative, framed within the conserving-approximation method of Baym and Kadanoff. Using as a benchmark the band gap of Si obtained in the GW approximation, together with a diagrammatic (and conserving) solution of the ensuing Bethe-Salpeter equation, we discuss issues involving conservation laws, self-consistency, and sum rules. These conceptual issues are particularly important for the development of ab initio methods for the study of dynamical response and quasiparticle band structure of strongly-correlated materials. We argue that inclusion of short-range correlations absent in the GW approximation is a must, even in Si.
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