While originally established for the calculation of nuclear-magnetic resonance (NMR) shielding tensors, the theoretical framework has also been extended to the calculation of NMR spin-spin coupling constant . be demonstrated that for this kind of system the large QM cluster size is essential to capture the environment effects on the hyperfine coupling constants. This strategy can even be pushed further in terms of relax-copy cycles for quasi-periodic molecular crystal models , as discussed in Section 5.4. In this way, spin densities (though no corresponding hyperfine coupling constants) have been calculated for a thiadiazolo-fused benzotriazinyl radical cluster model with more than 50,000 QM atoms in Ref. 12. While KS-DFT with standard approximations often shows the problem of spin-density overdelocalization for such systems, the corresponding subsystem DFT calculations may suffer from the opposite problem of a too strong spin-density localization. This can be alleviated in terms of the FDE-diab approach by forming linear combinations of several spin-localized configurations, as has been demonstrated for the calculation of hyperfine coupling constants in Ref. 319.