A first DMRG/CASSCF–CASPT2 study of a series of paradigmatic {FeNO}⁶, {FeNO}⁷, and {FeNO}⁸ heme–nitrosyl complexes has led to substantial new insight as well as uncovered key shortcomings of the DFT approach. By virtue of its balanced treatment of static and dynamic correlation, the calculations have provided some of the most authoritative information available to ...