Calculations of energy levels for atoms with several valence electrons

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 cole-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. (C) 1996 American Institute of Physics.