Vertical ionization energies of pyradine, pyridazine, pyrimidine, pyrazine, s-triazine, and s-tetrazine are calculated with partial third order electron propagator theory. Extensive reorderings of final states are produced by correlation corrections to Koopmans’s theorem results. The partial third order (P3) quasiparticle method succeeds in producing the correct order of final states and close agreement with photoelectron spectra. Because P3 is more efficient than the outer valence Green’s function and other methods based on the third order self-energy, it shows considerable promise for predicting photoelectron spectra of large molecules.