Theoretical study of the electronic spectrum of magnesium-porphyrin

Multiconfigurational self-consistent field (SCF) and second order perturbat ion methods have been used to study the electronic spectrum of magnesium-po rphyrin (MgP). An extended ANO-type basis set including polarization functi ons on all heavy atoms has been used. Four allowed singlet states of E-1u s ymmetry have been computed and in addition a number of forbidden transition s and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consis tently between 0.1 and 0.5 too low compared to the measured band maxima. Th e source of the discrepancy is the approximate treatment of dynamic correla tion (second order perturbation theory), limitations in the basis set and t he fact that all measurements have been made on substituted magnesium porph yrins. (C) 1999 American Institute of Physics. [S0021-9606(99)30814-X].