The potential energy curves along the Fe-N breathing mode, the most dominant normal mode of [Fe(terpy) 2 ] 2+ connecting the 1 GS (LS) and 5 MC (HS) states, are displayed in Figure . This represents a dimension along which the electronic transitions can be best interpreted, yet we need to take other modes into account that can couple the excited states. The potentials along one of such coupling modes, with antisymmetric Fe-N . Also shown is the molecular structure of the [Fe(terpy) 2 ] 2+ complex with "ax" and "eq" denoting the axial and equatorial nitrogen positions, respectively, as well as the 1 GS (LS) → 1 MLCT excitation process (∆E = 2.36 eV).