Abstract
We investigate charge transfer in prototypical molecular donor-acceptor compounds using hybrid density functional theory (DFT) and the GW approximation at the perturbative level (G0W0) and at full self-consistency (sc-GW). For the systems considered here, no charge transfer should be expected at large intermolecular separation according to photoemission experiment and accurate quantum-chemistry calculations. The capability of hybrid exchange-correlation functionals of reproducing this feature depends critically on the fraction of exact exchange $\alpha$, as for small values of $\alpha$ spurious fractional charge transfer is observed between the donor and the acceptor. G0W0 based on hybrid DFT yields the correct alignment of the frontier orbitals for all values of $\alpha$. However, G0W0 has no capacity to alter the ground-state properties of the system, because of its perturbative nature. The electron density in donor-acceptor compounds thus remains incorrect for small $\alpha$ values. In sc-GW, where the Green's function is obtained from the iterative solution of the Dyson equation, the electron density is updated and reflects the correct description of the level alignment at the GW level, demonstrating the importance of self-consistent many-body approaches for the description of ground- and excited-state properties in donor-acceptor systems.
Highlights
Donor-acceptor compounds have recently attracted considerable attention due to their application in the field of organic electronics [1,2]
TTF, TCNE, TCNQ, and p-chloranil, the mean absolute error (MAE) and mean error (ME) of the G0W0@PBEh(α) quasiparticle energies relative to photoemission experiment for the highest-occupied molecular orbital (HOMO) level and for the valence excitation spectrum are reported in Fig. 7 for α ∈ [0,1]
We investigated the reliability of density functional approaches, G0W0, and self-consistent GW approach (sc-GW) in describing groundand excited-state properties for a set of prototypical donoracceptor compounds
Summary
Donor-acceptor compounds have recently attracted considerable attention due to their application in the field of organic electronics [1,2]. First-principles methods that do not accurately capture orbital energies of systems may provide a qualitatively incorrect description of charge transfer and, subsequently, ground-state properties such as the charge density. The single-particle Green’s function provides a rigorous way to determine electronic excitations in molecules and solids and gives access to the total energy and the ground-state properties of a system. In this context, Hedin’s GW approximation [6] for the single-particle Green’s function has become a well-established framework for the calculation of IP and EA, referred to as quasiparticle excitations [7,8,9].
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