The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus rep
resent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories.
We study within the many-body Greens function GW and Bethe-Salpeter approaches the neutral singlet excitations of the zinctetraphenylporphyrin and C70 fullerene donor-acceptor complex. The lowest transition is a charge-transfer excitation between the
donor and the acceptor with an energy in excellent agreement with recent constrained density functional theory calculations. Beyond the lowest charge-transfer state, of which the energy can be determined with simple electrostatic models that we validate, the Bethe-Salpeter approach provides the full excitation spectrum. We evidence the existence of hot electron-hole states which are resonant in energy with the lowest donor intramolecular excitation and show an hybrid intramolecular and charge-transfer character, favouring the transition towards charge separation. These findings support the recently proposed scenario for charge separation at donor-acceptor interfaces through delocalized hot charge-transfer states.
We study within the many-body Greens function $GW$ and Bethe-Salpeter formalisms the excitation energies of several coumarin dyes proposed as an efficient alternative to ruthenium complexes for dye-sensitized solar cells. Due to their internal donor-
acceptor structure, these chromophores present low-lying excitations showing a strong intramolecular charge-transfer character. We show that combining $GW$ and Bethe-Salpeter calculations leads to charge-transfer excitation energies and oscillator strengths in excellent agreement with reference range-separated functional studies or coupled-cluster calculations. The present results confirm the ability of this family of approaches to describe accurately Frenkel and charge-transfer photo-excitations in both extended and finite size systems without any system-dependent adjustable parameter, paving the way to the study of dye-sensitized semiconducting surfaces.
We study using the Bethe-Salpeter formalism the excitation energies of the zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex. In great contrast with standard timedependent density functional theory calculations with (semi
)local kernels, charge transfer excitations are correctly located above the intramolecular Q-bands transitions found to be in excellent agreement with experiment. Further, the asymptotic Coulomb behavior towards the true quasiparticle gap for charge transfer excitations at long distance is correctly reproduced, showing that the present scheme allows to study with the same accuracy intramolecular and charge transfer excitations at various spatial range and screening environment without any adjustable parameter.
We study the electron-phonon coupling in the C60 fullerene within the first-principles GW approach, focusing on the lowest unoccupied t1u three-fold electronic state which is relevant for the superconducting transition in electron doped fullerides. I
t is shown that the strength of the coupling is significantly enhanced as compared to standard density functional theory calculations with (semi)local functionals, with a 48% increase of the electron-phonon potential Vep. The calculated GW value for the contribution from the Hg modes of 93 meV comes within 4% of the most recent experimental values. The present results call for a reinvestigation of previous density functional based calculations of electron-phonon coupling in covalent systems in general.
