On the relations between the polarization propagator, the principal propagator, and the particle‐hole propagator

It is demonstrated that the polarization propagator, the principal propagator, and in some cases also the particle‐hole propagator calculated through the same order in perturbation theory all yield the same excitation energies, while only the residues of the polarization can be identified with electronic transition moments in methods which go beyond the random‐phase approximation. The extra terms appearing in the second‐order polarization propagator approach account for up to 10% of the individual electronic transition moments.