minants in the final wave function, the CVS-uS approximation captures all channels involving Φ 0 (first four channels in part (i) and first three channels in part (ii) of Tables and). Significantly, the couplings of Φ 0 with the dominant singly core-excited configurations (CV) of the final wave function involve only the singly excited valence-or core-excited determinants as intermediates, which are captured in the CVS-uS approach.