  The electron-detachment energy is measured by its ionization potential (IP).
As a result, it is a fundamental observable and important molecular electronic
signature in photoelectron spectroscopy. A precise theoretical prediction of
electron-detachment energies or ionization potentials is essential for organic
optoelectronic systems like transistors, solar cells, or light-emitting diodes.
In this work, we benchmark the performance of the recently presented IP variant
of the equation-of-motion pair coupled cluster doubles (IP-EOM-pCCD) model to
determine IPs. Specifically, the predicted ionization energies are compared to
experimental results and higher-order coupled cluster theories based on
statistically assessing 201 electron-detached states of 41 organic molecules
for three different molecular orbital basis sets and two sets of particle-hole
operators. While IP-EOM-pCCD features a reasonable spread and skewness of
ionization energies, its mean error and standard deviation deviate up to 1.5 eV
from reference data. Our study, thus, highlights the importance of dynamical
correlation to reliably predict IPs from a pCCD reference function in small
organic molecules.
