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International audience; Achieving the ionization potentials of core electrons is becoming easier with the advances in X-ray spectroscopy, both the high energies become accessible and the precision improves [1]. The consideration of relativistic effects is necessary [2], so we are working to obtain these energies using 4-component Hamiltonians (Dirac-Coulomb / Dirac-Coulomb-Gaunt) by Equation of Motion Coupled Cluster (EOM-CCSD) implemented in the DIRAC code [3]. This method has many advantages both in terms of its precision and its predictions [4]. The size of the matrix (EOM) and the computational cost in N , real computational limitations, can be outwitted by the use of approximations, namely when calculating the reference wavefunction by neglecting some amplitudes T of the CC calculation (Frozen-Core (FC) [3]), or by limiting its size by the use of projectors (Core Valence Separation (CVS) [5,6] / No Double Excitation in Core (ND) [3]). This work shows the interest of applying these approximations and of testing their different degrees of precision in order to obtain the ionization energies of the core electrons. The studied systems are notably the highly relativistic core electrons in Xe, and {X-; HX (with X in {Cl, Br, I}) which have atmospheric interests.[1] Boudjemia N. et al., Phys. Chem. Chem. Phys. 21, 5448, (2019)[2] Fleig T. et al., Chem. Phys. 2 395 (2012); Saue T. et al., Chem. Phys. Chem. 12 3077, (2011)[3] Saue T. et al, J. Chem. Phys. 152; 204104, (2020)[4] Shee A. et al., J. Chem. Phys. 149, 174113, (2018)[5] Coriani S. et al., J. Chem. Phys 143, 181103, (2015)[6] Dreuw, A. et al., WIREs Comp. Mol S, 5:82-95, (2015)[7] Southworth S. H. et al., J. Chem. Phys, 142, 224302, (2015)[8] Oura M. et al. New J. Phys 21, 043015, (2019)[9] Sikkema J. et al.,J. Chem. Phys. 131, 124116 (2009)

National audience; Advances in X-ray spectroscopy, both in high-energy and in resolution, now allow us to reach the ionization potentials (IP) of the core electrons of many systems [3, 5]. In quantum chemistry, these electrons must be treated using relativistic Hamiltonians (Dirac-Coulomb, Dirac- Coulomb-Gaunt), as well as for electrons belonging to molecular orbitals (MO) having a strong Spin-Orbit coupling. In addition, the Ionization Potential - Equation of motion Coupled-Cluster (IP-EOM-CCSD) method has many advantages both by the precision of the results [6] and by its explanatory and predictive nature [2]. However, its computational cost in N6 quickly makes it inapplicable.We then explore the Core Valence Separation approximation (CVS-EOM- CCSD) [4] based on the flexible separation between core and valence MO to be taken into account in the EOM calculation. In parallel, we were interested in the Frozen Core (FC) approximation whose role is to make certain MO inactive during the preliminary Coupled-Cluster (CC) calculation. These different technics, recently implemented by our team in Dirac [1], have been applied to X- and HX systems (X : Cl, Br, I, At) and a systematic examination shows the advantage of using these different methods to obtain the IP of the core electrons.[1] DIRAC, a relativistic ab initio electronic structure program (available at http://dx.doi.org/10.5281/zenodo.3572669, see also http://www.diracprogram.org)[2] Bouchafra, Y., Shee, A., Réal, F., Vallet, V., and Gomes, A. S. P. Physical Review Letters 121, 2018, 26[3] Boudjemia, N., Jänkälä, K., Gejo, T., Nagaya, K., Tamasaku, K., Huttula, M., Piancastelli, M. N., Simon, M., and Oura, M. Physical Chemistry Chemical Physics 21, 2019, 5448.[4] Coriani, S., and Koch, H. The Journal of Chemical Physics 143, 2015, 181103.[5] Oura, M., Gejo, T., Nagaya, K., Kohmura, Y., Tamasaku, K., Journel, L., Pian-castelli, M. N., and Simon, M. Hard New Journal of Physics 21, 2019, 043015.[6] Shee, A., Saue, T., Visscher, L., and Gomes, A. S. P. The Journal of Chemical Physics 149, 2018, 174113.

The excited states of three radical derivatives of guanine, i. e. guanine cation (G+) and its two main deprotonated derivatives (G−H1 and G−H2), have been characterized in the Franck‐Condon region by TD‐DFT, using different functionals, CASPT2, and EOM‐EE‐CCSD calculations. In the gas phase, all the methods provide a similar description of the main spectral features, the pictures provided by TD‐DFT, with long range corrected functionals, and EOM‐EE‐CCSD being very close. Solvent effects are then taken into account by a mixed discrete‐continuum approach, including five water molecules of the first solvation shell and the Polarizable Continuum Model (PCM). The vibronic absorption line‐width has finally been simulated at the TD‐M052X level by a time dependent method within the harmonic approximation. The resulting absorption spectra are in good agreement with their experimental counterparts, providing useful indications on the use of PCM/TD‐DFT based approaches to interpret the spectra of guanine based radicals within DNA.

An extensive analysis has been carried out of the performance of standard families of basis sets with the hierarchy of coupled cluster methods CC2, CCSD, CC3, and CCSDT in computing selected Oxygen, Carbon, and Nitrogen K-edge (vertical) core excitation and ionization energies within a core-valence separated scheme in the molecules water, ammonia, and carbon monoxide. Complete basis set limits for the excitation energies have been estimated via different basis set extrapolation schemes. The importance of scalar relativistic effects has been established within the spin-free exact two-component theory in its one-electron variant (SFX2C-1e).