Hilde Fagerli

On the combination of ECP-based CI calculations with all-electron spin-orbit mean-field integrals

Abstract Spin-orbit interaction in ab initio calculations using effective core potentials is usually treated with pseudo-spin-orbit-operators, which are fitted on relativistic all-electron data. This procedure has been applied successfully for many systems. However, in combination with all-electron calculations, a recently developed atomic mean-field spin-orbit code succeeded in reproducing spectroscopic data with high accuracy. We propose a new way to combine these approaches in order to benefit from their conceptual and computational advantages. We present applications on two experimentally and/or theoretically well-known atomic systems, platinum and thallium, to prove the validity and precision of this new ansatz.

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

Abstract 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.

Spin-orbit effects in the PtH+2 ion

The five low-lying doublet states of PtH+2 have been studied by all-electron one- and two-component variational calculations using the Douglas–Kroll operator in a RASCI formalism. CASSCF calculations are performed in order to optimize orbitals for the subsequent RASCI calculations. Spin-orbit effects are introduced on the CI level both as a perturbation based on the spin-free CI states and variational in spin-orbit RASCI calculation. The influence of the spin-orbit effects are studied on geometries, excitation energies, reaction barrier and dissociation energies.

Extensive relativistic calculations on the palladium hydride molecule

The lower states of the PdH molecule have been studied with different relativistic methods. At the spin-free level, relativistic effects are shown to be crucial for a proper description of the molecule. PdH is shown to be a single-configuration molecule, but the configuration interaction (CI) results are highly sensitive to the choice of orbital basis. Spin–orbit interaction has been included through spin–orbit CI theory based on the second order Douglas–Kroll operator and through fully relativistic CI calculations based on the Dirac–Coulomb Hamiltonian. The results are in good agreement, but requires a proper choice of orbital basis in the spin–orbit CI calculation to ensure a balanced description of all ΛΣ states coupled by spin–orbit interaction.