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

摘要

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 (8s9p7d3f3g/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 d~10 vs d~9 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 d~10 vs d~9 correlation energy error between the ~2S-~2D and ~2S-~2P transition energies. For the neutral atom, the ~2D_5/2-~2D_3/2 and ~2P_3/2-~2P_1/2 splittings are in excellent agreement with the experimental ones. However, the relative calculated energetic ordering for the ~2D_5/2, ~2D_3/2, ~2P_3/2, and ~2P_1/2 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.