The problem of degenerate ground states of open-shell atoms is investigated in spin-restricted and spin-unrestricted density functional theories using the exact-exchange energy functional. For the spin-unrestricted case, spurious energy splittings of the order of 2-3 kcal/mol are found for atoms of the second and third periods which are larger than the splittings obtained from recently proposed approximate exchange functionals depending explicitly on the current density. In remarkable contrast, for spin-restricted calculations the degeneracy of different atomic ground states is recovered to within less than 0.6 kcal/mol.
Bibliographical noteFunding Information:
We gratefully acknowledge financial support through the Deutsche Forschungsgemeinschaft Priority Program 1145 “First-Principles Methods,” through the EU Network of Excellence NANOQUANTA, and through the EU Research and Training Network EXCITING.