Accurate starting points for one-shot $G_0W_0$ and Bethe-Salpeter Equation calculations via effective tuning of range-separated hybrid functionals

The accuracy of one-shot $G_0W_0$ and Bethe-Salpeter equation (BSE) calculations depends strongly on the underlying starting-point eigensystem, which is commonly obtained from a mean-field density-functional approximation. Range-separated hybrid (RSH) functionals provide a particularly effective starting point, however, conventional optimally tuned RSH procedures often require costly, system-specific, multi-step optimizations of the range-separation parameter $ω$. In this work, we show that a recently proposed effective tuning protocol [Singh \textit{et. al.}, Journal of Physical Chemistry Letters, 16, 32, 8198-8208, (2025)] for RSH functionals can serve as an efficient alternative for determining $ω$ used in $G_0W_0$ and BSE calculations. This simplified tuning scheme yields range-separation parameters that are effectively equivalent to those obtained from more elaborate tuning strategies, while avoiding their substantial computational overhead. The resulting tuned RSH eigensystems provide reliable starting points for many-body perturbation theory. In particular, one-shot $G_0W_0$ calculations based on effectively tuned RSH orbitals reproduce reference ionization potentials with high accuracy, while subsequent BSE calculations yield quantitatively reliable neutral excitation energies, optical absorption spectra, and excitonic properties for a diverse set of molecular systems and clusters. These results demonstrate that effective RSH tuning offers a practical and broadly applicable route to accurate quasiparticle and excited-state calculations, combining the accuracy of optimally tuned starting points with the low computational cost required for routine applications of $G_0W_0$ and BSE.