Accurate <i>ab initio</i> study on the spectroscopy of Ag and Ag+ including spin-orbit couplings aimed at molecular calculations

Abstract

Very accurate ab initio electronic + spin-orbit calculations of the lowest-lying states of the Ag atom and Ag+ cation have been performed through the CASSCF + ACPF + EPCISO method, using the Stuttgart small-core (19 active electrons) relativistic effective core potential (RECP) as well as its associated 2D spin-orbit effective potential. An ad hoc spin-orbit P-symmetry pseudopotential for the 2P state adapted to this 19-e RECP and basis set was extracted. The Stuttgart basis set was augmented to a large valence Gaussian basis set (8s8p7d3f3g/6s6p4d3f3g) in order to reproduce at best the experimental 2S-2D and 2S-2P transition energies as well as the ionization potential (IP) of Ag, which play a crucial role for the accurate description of the spectroscopy in silver-containing molecular systems. A detailed discussion on the multiple schemes used to deal with the differential d10 vs d9 electronic correlation for these two excited states is given. The role of the 4s and 4p (core) shells on the 2S-2D and 2S-2P transition energies and the IP is carefully studied and discussed. The core–core correlation is found to play a minor role while an insufficient treatment of the core-valence electronic correlation is responsible for the main differential d10 vs d9 correlation energy error between the 2S-2D and 2S-2P transition energies. For the neutral atom, the 2D5/2-2D3/2 and 2P3/2-2P1/2 splittings are in excellent agreement with the experimental ones. However, the relative calculated energetic ordering for the 2D5/2,2D3/2,2P3/2, and 2P1/2 fine structure components is critically dependent on the J-averaged purely electronic ACPF 2P and 2D energies of the parent states. The 3D fine-structure splitting for the ion is also found in good agreement with the experiment.