The energies of the S 0 and S 1 states of polychlorinated dibenzofurans (PCDFs) were calculated using the Hartree-Fock (HF) and configuration interaction-singles (CIS) methods. We can obtain the (0-0) transition energies of PCDFs with good accuracy if the energies calculated using the HF and CIS methods are adjusted to take the electron correlation energy into account. The correlation energy of the S 0 state was calculated using the Møller-Plesset correlation correction truncated at the second order (MP2), and that of the S 1 state was determined using experimental data. The correlation energies for both S 0 and S 1 states were expressed as the sum of the contributions arising from dibenzofuran (DF) and substituted chlorine atoms. The energy of the ground state calculated using the additivity approximation was in good agreement with the energy given directly by the MP2 method. The (0-0) S 1←S 0 transition energies corrected for electron correlation energy agreed well with the available experimental data. The approach proposed in this paper may be useful for the estimation of the electronic transition energy for large aromatic molecules.
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