High-Accuracy Calculations of X-ray Spectra via Relativistic Coupled-Cluster Techniques *

We present a systematic route to high accuracy for ab initio calculations of near edge x-ray absorption fine structure (NEXAFS) spectra using core-valence separated equation-of-motion coupled-cluster methods to systematically approach the full configuration interaction limit, an edge-specific scheme for capturing the diffuse nature of core-excited Rydberg states, and the exact two-component theory for treating relativistic effects. Benchmark calculations show that triple excitations not only play important roles in obtaining accurate absolute values for core excitation energies, but also make significant contributions to relative shifts between valence and Rydberg excitations. Calculations of gas-phase water molecule with the inclusion of quadruple excitations and adequate treatments of vibrational structures are shown to reach <0.2 eV accuracy in terms of absolute energies. We also report a joint experimental-computational study of the NEXAFS spectrum for ethyl-trifluoroacetate, as an example of a fairly complex molecule.