  The ionization potential (IP) is an important parameter providing essential
insights into the reactivity of chemical systems. IPs are also crucial for
designing, optimizing, and understanding the functionality of modern
technological devices. We recently showed that limiting the CC ansatz to the
seniority-zero sector proves insufficient in predicting reliable and accurate
ionization potentials within an IP equation-of-motion coupled-cluster
formalism. Specifically, the absence of dynamic correlation in the
seniority-zero pair coupled cluster doubles (pCCD) model led to unacceptably
significant errors of approximately 1.5 eV. In this work, we aim to explore the
impact of dynamical correlation and the choice of the molecular orbital basis
(canonical vs. localized) in CC-type methods targeting 201 ionized states in 41
molecules. We focus on pCCD-based approaches as well as the conventional
IP-EOM-CCD and IP-EOM-CCSD. Their performance is compared to the CCSDT
equivalent and experimental reference data. Our statistical analysis reveals
that all investigated frozen-pair coupled cluster methods exhibit similar
performance, with differences in errors typically within chemical accuracy (1
kcal/mol or 0.05 eV). Notably, the effect of the molecular orbital basis, such
as canonical Hartree-Fock or natural pCCD-optimized orbitals, on the IPs is
marginal if dynamical correlation is accounted for. Our study suggests that
triple excitations are crucial in achieving chemical accuracy in IPs when
modeling electron detachment processes with pCCD-based methods.
