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 1 A 1 + and 1 B 2 + 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.