The vertical electronic spectrum of pyrrole: A second and third order n-electron valence state perturbation theory study

The vertical electronic spectrum of pyrrole is investigated by means of second and third order n-electron valence state perturbation theory. The three 1A1-, 1B2+ and 1A1+π→π∗ valence states, as well as the 3s, 3p and 3d π- and σ-type Rydberg states, are considered. Particular attention is paid to the description of the valence states, where different active spaces of increasing size are used to improve the zero order wave function. For the Rydberg states and the covalent valence state (1A1-), the perturbative results show a coherent trend and are in accordance with those of the previous high-level studies. For the two ionic valence states (1B2+ and 1A1+), rather large active spaces are required to get satisfactory results.