Vertical spin-state energy gaps for the dimer at the various experimental (XRD) geometries were computed using multireference methods (hydrogen positions were not determined from the XRD data, but were instead optimized at the BP86/def2SVP level). To refer to these geometries, we adopt the nomenclature presented in Table . The first subscript refers to the temperature at which the XRD experiment was conducted and, when needed, a Roman numeral is added to distinguish the two distinct dimeric units in the unit cell (cf. Figure ).