Structural diradical character

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, and triple bonds, modeling electronic diradical character requires accurate bond-length patterns. A generally applicable measure for comparing first-principles bond-length patterns with experimental ones is not available at present. For this purpose, we introduce a new measure for structural (rather than electronic) diradical character. It has the advantage of being easily applicable to both computed and experimental molecular structures for arbitrary systems. Within Kohn-Sham density functional theory, we employ this measure for evaluating the suitability of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh) for describing experimentally characterized organometallic and organic structures at the diradical / closed-shell border. The two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic reference 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. This underlines the importance of structural diradical character as a test for the further evaluation and development of electronic structure methods.