In X2PA transitions (Fig. ), the final states are core-excited states. Such states are highlying and, therefore, difficult to compute using standard numerical solvers. Moreover, these states are Feshbach resonances embedded in the valence ionization continuum. As a result, standard correlated methods designed for computing electronically bound states cannot be used for these resonances, whose wave functions are not L 2 -integrable. Without special modification to these methods, attempts to compute core-excited states are plagued by the erratic (often, divergent) behavior of solvers, a lack of systematic convergence with basis-set increase, and often, unphysical solutions. 12, This issue is effectively addressed by the corevalence separation (CVS) approach 71 in which the core resonances are decoupled from the valence continuum by removing the purely valence excited determinants from the EOM-CC configuration space. This pruning of the Fock space makes the core-level states artificially bound in the computation, leading to convergent results. The errors in energies introduced due to the CVS dilution of dynamical correlation in meth-Figure : In X2PA, two X-ray photons of energies ω 1 and ω 2 are coherently absorbed such that ω 1 +ω 2 equals the excitation energy E f -E g of the final state f that is core excited relative to the initial state g. The X2PA moments have contribution from all states of the system, including the ground and bound valenceexcited states (black lines), core-excited states (blue lines), valence resonances (magenta lines), and valence continuum states (ultrafine gray dashes). ods such as EOM-CC are typically of the order of less than 1 eV. The effect of the continuum can be reincorporated, e.g., via a perturbative approach or via Feshbach-Fano formalism. Further, the CVS treatment is not state specific so that multiple states can be computed simultaneously, just as in the case of standard EOM-CC calculations; this feature is attractive in the context of spectroscopy modeling.