ELECTRON CORRELATION IN TETRAPYRROLES - AB-INITIO CALCULATIONS ON PORPHYRIN AND THE TAUTOMERS OF CHLORIN

Ab initio calculations including geometry optimization at the Hartree-Fock, MP2, and LDF levels have been carried out for free-base porphyrin and for the tautomers of free-base chlorin, using polarized basis sets. The Hartree-Fock approximation artificially favors frozen resonance structures with alternating single and double bonds for these compounds. This incorrect behavior is completely reversed when correlation effects are accounted for. Correct, delocalized structures of tetrapyrroles are obtained with the MP2 and LDF levels of approximation.