Ab initio CI calculations on free‐base porphin

Ab initio configuration interaction ( CI ) calculations were carried out on low‐lying singlet and triplet π–π* states and ionized states of free‐base porphin. We take into account single and double excitations from σ and π electrons in the CI calculations. The composite natural orbitals were employed in order to reduce the size of orbital set to be used in the CI . The calculated excitation energies were in good agreement with experimental values. The use of split‐valence‐type basis and the inclusion of correlation effects of σ electrons were proved to be important to describe the low‐lying π–π* states, especially the Soret band.