This is known as the Tamm-Dancoff approximation (TDA), and it is typically accurate for excitation energies. Moreover, the TDA decouples the excitation energy problem from the ground-state stability problem, which is sometimes necessary in photochemical simulations. On the other hand, the TDA violates gauge invariance and thus introduces ambiguity for oscillator strengths, which are typically more accurate in full linear response theory. In any case, the CT metrics introduced below can be used with either eq. 2 or eq. 3, although all calculations reported here are full linear response.