Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of pyrrole, furan, and cyclopentadiene

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Abstract
The symmetry-adapted cluster (SAC) expansion and the SAC-CI theory have been applied to the calculations of the valence and Rydberg excitations and ionizations of the five-membered ring compounds, pyrrole, furan, and cyclopentadiene. For almost all cases, the experimental values were reproduced to within 0.3 eV for the excitation energy, and to within 0.5–0.7 eV for the ionization potential. We have given several new assignments for the excitation spectra of pyrrole and furan, and the first ab initio assignments for cyclopentadiene. However, there were remarkable disagreements of about 1.2 eV for the singlet π → π* excitations of 2 1A1 state for furan, and of 1 1B2 state for cyclopentadiene. This is the same tendency as those due to the MRSD-CI method. For ionization potential, our assignments of the peaks are essentially the same as those due to the Green’s function method by von Niessen et al., but the present SAC-CI method reproduces the experimental data better than this method.

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