AB INITO CALCULATIONS OF THE P-2(1 2)-P-2(3/2) SPLITTING IN THE THALLIUM ATOM/

The splitting between the P-3(3/2) and the P-2(1/2) terms in the thall ium atom has been calculated at the perturbation theory level and by s pin-orbit CI calculations, using both the Breit-Pauli asci the no-pair form of tile microscopic spin-orbit Hamiltonian, The importance of th e spin-other-orbit contribution to the spin-orbit splitting is investi gated. and it is also shown that an averaging procedure of the kinemat ic factors in the expression for the spin-other-orbit integrals in the no-pair spin-orbit Hamiltonian yields highly accurate results. A slig htly modified version of a previously proposed mean-field spin-orbit m ethod is shown to have an accuracy of a few wave numbers. Perturbation theory is found to give a term-splitting which is too low by more tha n 1000 cm(-1), while spin-orbit CI with the no-pair form of the spin-o rbit operator with the averaged spin-other-orbit term, and the no-pail mean-field operator, gives results in good agreement with experimenta l data.