(a) Spin state energetics. Ever since density functional theory gained a widespread following among chemists, especially experimental chemists, the question of spin state energetics of transition metal complexes has been a vexing one. In early studies, we (as well as others) showed that classic pure functionals often exhibit an undue preference for lower-spin states, and hybrid functionals err in the other direction, favoring higher-spin states. In particular, we found the spin-crossover complex and nitrosylheme analogue Fe(salen)(NO) (as well as other spin-cross-over complexes ) to serve as a particularly useful test case for a functional's performance vis-à-vis spin state energetics. Now, by accounting for both static and dynamic correlation in a balanced manner, state-of-the-art DMRG-CASSCF/CASPT2 calculations provide (to our knowledge) the most authoritative adiabatic low-high spin state gaps for the series of paradigmatic {FeNO} 6-8 complexes examined here (see also other examples of multireference studies of transition metal spin state energetics ). The present results fill a key gap in the nitrosyl literature, since, as of now, comparably accurate results are only available for Fe[P](NO) and Fe[C](NO), where P and C refer to unsubstituted porphine and corrole, respectively. The main findings are as follows. the Fe carries about four-fifths of the spin density, with most of the remaining fifth on the NO, reflecting the effect of the antibonding Fe(dz2)-ImH antibonding interaction. As shown in Figures and, pure functionals largely capture the essence of the DMRG spin density profile, whereas hybrid functionals lead to much greater spatial separation of the majority and minority (alternatively, up and down) spin densities. For the singlet {FeNO} 6 and {FeNO} 8 species, DMRG-CASSCF calculations "by definition" indicate zero spin density at every point, in contrast to DFT, which results in various degrees of spin symmetry-breaking, from negligible for classic pure functional to pronounced for hybrid functionals. The fact that the large Fe spin density in the {FeNO} 7 state is neutralized in the {FeNO} 6 and{FeNO} 8 states may be naively regarded as indicative of essentially metal-centered oxidation or reduction, respectively. It is worth recalling that early UV-vis spectroelectrochemical studies of simple {FeNO} 7 porphyrins by Kadish and coworkers also reached similar conclusions, i.e., FeNO-centered redox processes. Below we shall see that an analysis of the DMRG wave function adds considerable detail to the above qualitative analyses.