<h3>Title</h3>
<h3>A paremeter-free approach for core-level spectroscopy without core-hole reference states</h3>
<p><strong><span style="text-decoration: underline;">Abstract</span> :</strong></p>
<p>Element-specific inner shell spectroscopy provides valuable information about the local chemical environment of atoms in molecules. Advanced light sources have made it possible to probe the inner shells of heavier elements leading to an increasing number of experiments. A balanced description of orbital-correlation and -relaxation effects, and relativistic corrections is needed for such theoretical studies. The importance of relativistic effects becomes prominent for elements heavier than Ne. Here, we combine GKS-spRPA with scalar relativistic (sr) X2C to study the X-ray photoelectron spectroscopy (XPS) of molecules of third period elements. This method produces MAEs of less than an eV for the core 1s and 2s level of third-period elements. We also developed a method for simulating X-ray emission spectroscopy (XES) where the emission energies are estimated as a difference between the valence and core ionization eigenvalues, computed using the sr-GKS-spRPA. The sr-GKS-spRPA one-particle eigenvalues provide accurate valence and core ionization energies using which accurate XE energies can be estimated without any need for core-hole reference states (unlike ∆SCF methods), empirical shifting parameters, or tuning of density functionals. The sr GKS-spRPA XE energies are within 0.2 eV of experimental values for a set of 27 emission energies of molecules containing second-period and third-period elements. The analytically continued (AC) GKS-spRPA approach, with a O(N<sup>4</sup> ) computational cost, enables efficient computation of medium-sized molecules. GKS-spRPA method can be used straightforwardly for simulating the XES spectra of symmetric molecules like C<sub>6</sub>H<sub>6</sub> and C<sub>60</sub> with several core-hole states.</p>
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