While the dipole moment D only depends on the occupied orbitals (within the DFT framework), a proper account of electron-vibration coupling should include the unoccupied orbitals as well. In 1 •+ , we find that the singly unoccupied molecular orbital (SUMO) is also strongly coupled to movement along several vibrations, resulting in a very large ΔEgap (Figure ) at the IRAV positions. Therefore, a large ΔEgap may be used as a diagnostic to identify IRAVs and vibrational modes strongly coupled to electronic states, despite being calculated at the equilibrium geometry.