Electronic excitation spectra of furan and pyrrole: Revisited by the symmetry adapted cluster-configuration interaction method

Electronic excitation spectra of furan and pyrrole are reinvestigated by th e symmetry-adapted cluster configuration-interaction method. The 47 and 46 lowest singlet and triplet electronic states are computed for furan and pyr role, respectively. Two series (1a(2) and 2b(1)) of low-lying Rydberg state s and the valence pi-pi* excited states strongly influence each other in bo th furan and pyrrole. The present calculations give detailed and satisfacto ry theoretical assignments of the vacuum ultraviolet spectra and the electr on energy-loss spectra of the two molecules. The similarities and differenc es in the electronic excitations between furan and pyrrole are discussed in detail. The accuracy and assignments of recent theoretical studies, i.e., complete active space second-order perturbation, multireference Moller-Ples set perturbation, second-order algebraic-diagrammatic construction, multire ference double configuration interaction, and CC3, are compared. (C) 2000 A merican Institute of Physics. [S0021-9606(00)30842-X].