All-electron Dirac-Coulomb and RECP calculations of excitation energies   for mercury atom with combined CI/MBPT2 method
  Calculations of transition energies between low-lying states of mercury atom are performed in the frame of combined CI/MBPT2 method. Results of all-electron relativistic calculations (using the Dirac-Coulomb Hamiltonian) are compared with experimental data and results of other four-component calculations. The results of the RECP calculations are compared with the corresponding all-electron results in order to estimate accuracy of different RECPs. Contributions from correlations in different shells to the calculated excitation energies as well as effects of basis set truncation at different orbital angular momenta, nuclear models, errors in gaussian approximation of the GRECP components are reported. Analysis of the obtained results shows that at least 34 external electrons of mercury atom should be correlated and the one-electron basis set should contain up to h angular momentum functions in order to reach a reliable agreement with experimental data within 200 1/cm. It is concluded that correlations of the 4f electrons can be efficiently taken into account for 20 electron GRECP at the generation stage. 
N. Mosyagin, M. Kozlov, and A. Titov (Petersburg Nuclear Physics   Institute)