An application of second and third-order n-electron valence state perturbation theory to the calculation of the vertical electronic spectrum of furan

The vertical electronic spectrum of furan is investigated by second and third-order multireference perturbation theory (NEVPT). The excitation energies of the three lowest-energy valence states, as well as the 3l Rydberg states are reported. The effects of the size of the active space and the valence-Rydberg mixing are discussed. The application of the quasi-degenerate NEVPT approach has proved to be necessary in order to handle the consistent valence-Rydberg interactions occurring for the two 1A1+ and 1B2+ valence states. For the three valence states and the low-lying Rydberg states, the computed excitation energies agree with those computed in the more recent high-level theoretical studies.