The ground, excited, and ionized states of free-base porphin (FBP) are reinvestigated by the SAC (symmetry adapted cluster)/SAC-CI (configuration interaction) method. In particular, the Rydberg excited states, the valence-Rydberg mixing, and the effect of polarization d-functions are studied. The lowest Rydberg excited state would exist at around 5.5 eV with a weak intensity, but the valence-Rydberg mixing is small. The effects of both the Rydberg and polarization d-functions are relatively small for the excitation energies of the main peaks, whereas the polarization d-functions affect considerably the ionization spectrum.