Optical spectra from molecules to crystals: Insight from many-body perturbation theory

Abstract

Time-dependent density-functional theory (TDDFT) often successfully reproduces excitation energies of finite systems, already in the adiabatic local-density approximation (ALDA). Here we show for prototypical molecular materials, i.e., oligothiophenes, that ALDA largely fails and explain why this is so. By comparing TDDFT with an in-depth analysis based on many-body perturbation theory, we demonstrate that correlation effects crucially impact energies and character of the optical excitations not only for molecules of increasing length and in crystalline environment, but even for isolated small molecules. We argue that only high-level methodologies, which explicitly include correlation effects, can reproduce optical spectra of molecular materials with equal accuracy from gas phase to crystal structures.

Electronic and Optical Properties of Oligothiophene-F4TCNQ Charge-Transfer Complexes: The Role of the Donor Conjugation Length

“…For further details about this formalism, see, e.g., Refs. 29,33,35. We solve the BSE in the Tamm-Dancoff approximation (TDA), which does not impact the nature of the excitations analyzed below.…”

Optical properties of azobenzene-functionalized self-assembled monolayers: Intermolecular coupling and many-body interactions

“…Remarkably, a qualitatively similar behavior is exhibited by the core- level excitations from the nitrogen K-edge of azobenzenefunctionalized SAMs. 63 In that case, the binding energy of the first bound exciton is reduced by about 2 eV going from the isolated molecule to the p-SAM. Another important effect of intermolecular interactions is the redistribution of the spectral weight to higher energies, due to dipole coupling.…”

“…33,34 This methodology allows for describing optical excitations in molecular materials from the gasphase to the condensed matter. [35][36][37][38][39][40][41][42] In matrix form, the BSE is expressed as…”

Addressing electron-hole correlation in core excitations of solids: An all-electron many-body approach from first principles

“…As discussed in Refs. [57,60,61] for optical spectra, in this case the screened Coulomb interaction merely induces a rigid red-shift of the IPA spectrum. Indeed, the resulting spectrum shown in Fig.…”

“…However, most of the schemes proposed so far are either based on pseudo-potential approximation [21,22], or, in an all-electron framework, on the description of the core states in terms of local orbitals as part of the valence region, neglecting spin-orbit coupling. While the latter choice turned out to be effective in many cases [18,[23][24][25][26], this approximate treatment of core states is limited to a specific energy window and initial states. In particular, excitations from core levels with a non-negligible spin-orbit splitting need a relativistic treatment, even for low transition energies.…”