Calculations of energy levels for atoms with several valence electrons

Abstract
A new ab initio method for doing high-accuracy calculations for atoms with more than one valence electron is described. An effective Hamiltonian for the valence electrons is formed using many body perturbation theory for the residual core-valence interaction. The configuration interaction method is used then to find the energy levels of the atom. Application to thallium shows that this 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.