Exploiting a derivative discontinuity estimate for accurate $G_0{W}_0$ ionization potentials and electron affinities

The $GW$ approximation has become an important tool for predicting charged excitations of isolated molecules and condensed systems. Its popularity can be attributed to many factors, including a favorable scaling and relatively good accuracy. In practical applications, the $GW$ is often performed as a one-shot perturbation known as $G_0{W}_0$. Unfortunately, $G_0{W}_0$ suffers from a strong starting point dependence and is often not as accurate as one would need. Self-consistent $GW$ methodologies alleviate these problems but come with a marked increase in computational cost. In this manuscript, we propose the use of an estimate of the exchange-correlation derivative discontinuity to provide a remarkably good starting point for $G_0{W}_0$ calculations, yielding ionization potentials and electron affinities with eigenvalue self-consistent $GW$ quality at no additional cost. We assess the quality of the resulting methodology with the GW100 benchmark set and compare its advantages over other similar methods.