Pantzasis 362 pointed out that Kohn-Sham DFT generally fails to reproduce the experimental spin ladder for such systems (unless some ad-hoc spin purification is performed, and thus the magnetic coupling constant, and a more physical representation of the low-spin states requires a multireference treatment based on restricted-active-space SCF (RASSCF) or DMRG wave functions. However, also within the context of a large active space, a post wave-function treatment is needed. Multireference pair-density functional theory (MC-PDFT) starting from a large active space (DMRG or RASSCF active space with 30 electrons in 22 orbitals) gives encouraging results. 363 (3.4.7) Kaupp and Arbuznikov : In the context of the failures for multi-reference cases, it seems important to mention attempts to account for strong-correlation effects and minimize fractional-spin errors. An important direction of development are Becke's real-space models of nondynamical correlation (initially the B05 functional ), which have been extended to account for strong-correlation terms by relying on the adiabatic connection (Becke's B13 model and the related KP16/B13 model 169 by Kong and Proynov). One interesting aspect of these functionals is that they are based on full exact exchange and model nondynamical correlation without using semilocal exchange.