  Theoretical descriptions of excited states of molecular systems in
high-energy regimes are crucial for supporting and driving many experimental
efforts at light source facilities. However, capturing their complicated
correlation effects requires formalisms that provide a hierarchical
infrastructure of approximations. These approximations lead to an increased
overhead in classical computing methods, and therefore, decisions regarding the
ranking of approximations and the quality of results must be made on purely
numerical grounds. The emergence of quantum computing methods has the potential
to change this situation. In this study, we demonstrate the efficiency of
Quantum Phase Estimator (QPE) in identifying core-level states relevant to
x-ray photoelectron spectroscopy. We compare and validate the QPE predictions
with exact diagonalization and real-time equation-of-motion coupled cluster
formulations, which are some of the most accurate methods for states dominated
by collective correlation effects.
