Combination of the many-body perturbation theory with the configuration-interaction method

An ab initio method for high accuracy calculations for atoms with more than one valence electron is described. The effective Hamiltonian for the valence electrons is formed using many-body perturbation theory for the interaction of the valence electrons with the core. The configuration-interaction method is then used to find the energy levels of the atom. An application of this to thallium shows that the method gives an accuracy of about 0.5% for the ionization potential and a few tenths of a percent for the first few energy intervals.