Keeping in mind that hydrogen-bonding in the solid state makes comparison between CASPT2 and experiment qualitative, the electronic absorption spectrum suggests that the Cr-Cr stretching mode lies between 150 and 250 cm -1 . FHVA CASPT2 predicts two modes with ν(Cr 2 ) character at 162 and 276 cm -1 . PHVA yields similar results with modes at 196 and 266 cm -1 (Table ). On the other hand, DFT (M06) predicts this mode at 585 cm -1 ν(Cr 2 ), consistent with an unreasonably short Cr-Cr distance.