The role of QED effects in transition energies of heavy-atom alkaline earth monofluoride molecules: a theoretical study of Ba$^+$, BaF, RaF and E120F

Abstract: Heavy-atom alkaline earth monofluoride molecules are considered as prospective systems to study spatial parity or spatial parity and time-reversal symmetry violating effects such as the nuclear anapole moment or the electron electric dipole moment. Comprehensive and highly accurate theoretical study of the electronic structure properties and transition energies in such systems can simplify the preparation and interpretation of the experiments. However, almost no attempts to calculate quantum electrodynamics (QED) effects contribution into characteristics of neutral heavy-atom molecules have been performed. Recently, we have formulated and implemented such an approach to calculate QED contributions to transition energies of molecules [L.V.~Skripnikov, J. Chem. Phys. \textbf{154}, 201101 (2021)]. In this paper, we perform a benchmark theoretical study of the transition energies in the Ba$^+$ cation and BaF molecule. The deviation of the calculated values from the experimental ones is of the order 10 cm$^{-1}$ and is more than an order of magnitude better than the "chemical accuracy", 350 cm$^{-1}$. The achievement of such an agreement has been provided in particular by the inclusion of the QED effects. The latter appeared to be not less important than the high-order correlation effects beyond the coupled cluster with single, double, and perturbative triple cluster amplitudes level. We compare the role of QED effects for transition energies with heavier molecules -- RaF and E120F, where E120 is the superheavy Z=120 homolog of Ra.