Rydberg character of the higher excited states of free-base porphin

The Rydberg character of the excited states of free-base porphin (FBP) has been investigated by the ab initio configuration interaction singles (CIS) method and the state-averaged complete-active-space self-consistent-field m ethod. Double-zeta basis sets augmented with s, p, and d Rydberg functions and d polarization functions have been employed. Two types of molecular orb itals sets, the restricted Hartree-Fock molecular orbitals obtained for the ground state ((1)A(g)) and for the cation state ((2)A(u)), have been used in the CIS calculations. All the calculations show that Rydberg-type excita tions play important roles especially in the N bands. In this article we pr opose applying the model of a perturbed Rydberg series to interpret the exc ited states of FBP. By using this model, we have succeeded in analyzing the characteristics of the excited states as well as the experimental oscillat or strengths, which have considerable magnitude even in the higher excited states.