Towards an accurate treatment of σ∗ ← σ transitions: Moving onto N6.-

- We report high level ab initio multi reference calculations of the excitation energies of a 3e-2c system.
- The excitation energy is extremely sensitive to the ground state equilibrium geometry.
- Only MRCI using a large active space is able to provide good results for the excitation energy.
- A full QM Born-Oppenheimer molecular dynamics is reported for the first time for a 3e-2c system.
- We estimate vibronic effects on the absorption spectrum of the hemi-bonded N6.-.