  A reliable first-principles description of singlet diradical character is
essential for predicting nonlinear optical and magnetic properties of
molecules. Since diradical and closed-shell electronic structures differ in
their distribution of single, double, triple and aromatic bonds, modeling
electronic diradical character requires accurate bond-length patterns, in
addition to accurate absolute bond lengths. We therefore introduce structural
diradical character, which we suggest as an additional measure for comparing
first-principles calculations with experimental data. We employ this measure to
identify suitable exchange-correlation functionals for predicting the bond
length patterns and electronic diradical character of a biscobaltocene with the
potential for photoswitchable nonlinear optical activity. Out of four popular
approximate exchange-correlation functionals with different exact-exchange
admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and
B3LYP perform best for diradical bond length patterns, with TPSSh being best
for the organometallic validation systems and B3LYP for the organic ones.
Still, none of the functionals is suitable for correctly describing relative
bond lengths across the range of molecules studied, so that none can be
recommended for predictive studies of (potential) diradicals without
reservation.
