On the efficiency of an effective Hamiltonian in spin-orbit CI calculations

All-electron spin-orbit-CI calculations based on the pseudo relativistic no-pair formalism have proven to give an adequate description of the non-additive spin-orbit and correlation effects. Although methodological and computational improvements have been achieved, the lack of size consistency and the dimension of the Hamilton matrix were found to be limitations for obtaining higher accuracy on compounds with heavy elements. The use of an effective Hamiltonian at the pseudopotential level has been proposed to take account of dynamic correlation effects at the spin-free level and to transfer this information to the iterative spin-orbit CI scheme. We have implemented this approach into a direct all-electron SO-CI code. Applications to the platinum atom and to TlH have been carried out.