The structures of 1a and 1b were optimized at both the B3LYP/ZORA-def2TZVP and BP86/ZORA-def2TZVP level with a S = 2 ground state. In all cases the optimized structures produced bond parameters in close agreement with the crystal structure data (Table ). For 1a, the primary difference between the optimized structure and the solid-state structure is the orientation of the Fp moiety, which rotates 33° around the Fe-Fe axis. As previously noted, the 1 H NMR data indicate free rotation about the Fe-Fe bond and so this difference likely arises from a low-energy torsion that is exerted by crystal packing effects. The B3LYP functional more accurately reproduced the Mössbauer isomer shift parameters over BP86 and was used for all further calculations (Table ). It should be noted that the calculated quadrupole splitting values for the three-coordinate iron site were far from the experimental observations, regardless of method used (e.g. 1a Fe2 |DEQ| = 0.90 (experimental), 2.03 (B3LYP), 1.85 (BP86) mm s -1 ).