A new challenge for time-dependent density functional theory

The quantum defect is an excellent tool to study errors in excitation energies obtained with ab initio methods. We perform a quantum defect analysis of the s, p, and d Rydberg states of Be with accurate ground state Kohn–Sham potentials. The s- and p-quantum defects are well described by the ALDA, but it fails badly for the d-quantum defect. This failure is not clear from the excitation energies alone and it provides a new challenge for functional development in time-dependent density functional theory.