Size-dependence of non-empirically tuned DFT starting points for $G_0W_0$ applied to ${\pi}$-conjugated molecular chains

Abstract

$G_0W_0$ calculations for predicting vertical ionization potentials (IPs) and electron affinities of molecules and clusters are known to show a significant dependence on the density functional theory (DFT) starting point. A number of non-empirical procedures to find an optimal starting point have been proposed, typically based on tuning the amount of HF exchange in the underlying hybrid functional specifically for the system at hand. For the case of $\pi$-conjugated molecular chains, these approaches lead to a significantly different amount of HF exchange for different oligomer sizes. In this study, we analyze if and how strongly this size dependence affects the ability of non-empirical tuning approaches to predict accurate IPs for $\pi$-conjugated molecular chains of increasing chain length. To this end, we employ three different non-empirical tuning procedures for the $G_0W_0$ starting point to calculate the IP of polyene oligomers up to 22 repeat units and compare the results to highly accurate coupled-cluster calculations. We find that, despite its size dependence, using an IP-tuned hybrid functional as a starting point for $G_0W_0$ yields excellent agreement with the reference data for all chain lengths.