An extrapolation scheme for spin–orbit configuration interaction energies applied to the ground and excited electronic states of thallium hydride Original Research Article

All-electron relativistic calculations have been performed on the 0+ ground state of TlH and all excited electronic states that dissociate to H(2S) and the 1/2(I) and 3/2(I) components of Tl(2P). Molecular orbitals are optimized employing a spin-free no-pair Hamiltonian while spin–orbit coupling is taken into account subsequently by one of the following approaches: (a) quasi-degenerate perturbation theory on the basis of multi-reference configuration interaction wavefunctions; (b) spin–orbit configuration interaction; and (c) spin–orbit configuration interaction combined with a perturbation theory estimate of the electron correlation contributions from discarded configurations. Spin-polarization is found to have a significant influence on the spectroscopic parameters of the ground and excited electronic states. Results in most satisfactory agreement with experiment are obtained, if approach (c) is applied.