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The newly developed similarity transformed equation-of-motion coupled-cluster (STEOM-CC) method and its perturbative analog (STEOM-PT) are applied to obtain ionization potentials, electron affinities and singlet and triplet excitation energies of free base porphin. STEOM-CC results are found to agree nicely with the available experimental data. Ground-state correlation is not described adequately in lowest order of perturbation theory, although STEOM-PT results qualitatively agree with the infinite order STEOM-CC results. The STEOM computational strategy is operationally like CI singles for excited states, which leads to exceptional efficiency.
Nooijen et al. (Tue,) studied this question.