Can Electron Propagator Methods Be Used To Improve Polarization Propagator Methods?

Calculations of Rydberg excitation energies with the second-order polarization propagator approximation (SOPPA) often produce results which are more in error than the random phase approximation (RPA), which formally is the first-order model. This is obviously because of cancellation of errors at the RPA level. On the other hand, valence excitation energies behave as expected, and they are systematically improved in SOPPA compared to RPA. Note that a Rydberg series is related to one of the ionization thresholds of the molecule, and it is thus obvious that a good description of the ionization limits is necessary in order to calculate good values for the Rydberg excitations. From perturbative electron propagator methods it is well-known that the second-order level is inadequate to obtain good ionization energies. It is also known from electron propagator methods that partial inclusion of higher-order terms can greatly improve the ionization energies. In this work it will be investigated if the lessons from electron propagator models can be used to improve to the calculation of Rydberg excitations in perturbative polarization propagator methods.