Transition metals compounds: Outstanding challenges for multiconfigurational methods

The present contribution provides a “perspective” on the work on transition metal (TM) bonding and spectroscopy by Björn O. Roos, performed by means of the CASPT2 method in the period 1992–2010. After giving a short overview of important nondynamic correlation effects appearing in TM atoms and molecules, ample attention is given to the work on the Cr 2 molecule and to Björn's fascination for multiple bonds in inorganic molecules containing TM. A short description is also given of his contributions to bioinorganic chemistry, with emphasis on the blue copper proteins.