where x I ia and y I ia are the usual LR excitation (x I ia ) and de-excitation (y I ia ) amplitudes for the Ith excited state. If the Tamm-Dancoff approximation (TDA) is invoked, 3 corresponding to neglecting the de-excitation amplitudes, then x I ia coincides with c I ia in Eq. ( ). Importantly, however, while the expressions for h CIS 0I and h DFT 0I are formally very similar, the latter might be nonzero even at a crossing point. This is more clear in the complete-basis limit, where h DFT 0I reduces to the Chernyak-Mukamel expression,