Towards an accurate treatment of σ∗ ← σ transitions: Moving onto

Dimeric σ∗ radical anions are ubiquitous, and their formation, spectroscopy and outcome can often be elucidated by density functional theory. But for shorter interfragment distances, three-electron two-center systems can be reluctant to a single-determinant description, such as the hexanitrogen radical anion. For N 6 . - , we show that multireference configuration interactions calculations are required to recover its characteristic electronic excitation energy, while TDDFT fails even with modern exchange–correlation functionals. The effects of vibronic couplings on the absorption spectrum are delineated based on a full quantum mechanical dynamical treatment; this study opens the door towards an accurate description of the subtle solvatochromism of hemi-bonded systems.