Estimating vertical core-excitation energies from Møller–Plesset theory with spin projection

Spin projected ΔUHF and ΔUMPn (n = 2, 2.5, 3) methods are used to calculate vertical core excitation energies. These methods are applied to a set of symmetrical molecules with equivalent atoms. We assess the role of core localisation, SCF orbital relaxation, pair correlation, and different relativistic corrections on the accuracy and reliability of the results. Additionally, we explore the limitations of using core-hole reference determinants and address some complications that may arise in perturbative calculations.