A Near-FCI Framework for Vibrationally Resolved X-ray Absorption Spectra.

We present a near-full configuration interaction framework based on iterative configuration interaction with selection and second-order perturbation theory (iCIPT2) for simulating vibrationally resolved X-ray absorption spectra. By combining high-fidelity potential energy curves with quantum wavepacket dynamics, our approach captures strong correlation and nuclear dynamics. Benchmarking on six diatomic systems yields sub-0.1 eV accuracy and quantitative agreement with high-resolution experiments. This work demonstrates a promising approach for interpreting vibrationally resolved core-level spectroscopy, established here for diatomic systems as an initial step, and lays a theoretical foundation for applications in more-complex ultrafast experiments at large-scale X-ray facilities.