The validity of approaches for calculating J-coupling using broken-symmetry HF and DFT solutions has been examined in detail in prior work. The quality of both SP and NP, in particular, depends on the magnitude of spin coupling. In the weak coupling regime, where the overlap between interacting magnetic orbitals a and b is close to zero, i.e. a|b ≈ 0, SP should be chosen. NP tends to perform better in the strong coupling regime, where a|b ≈ 1. However DFT tends to overestimate the magnitude of J-couplings, 56 a result that has been explored by other authors. In practice, NP tends to underestimate the magnitude of J, and when used with DFT, gives better results even in the weak coupling regime. Alternatively, the AP model smoothly unifies the two limiting cases treated by the SP and NP approaches. The AP model employs the dominant HS contaminant, finds its weight in the broken-symmetry state, removes it from the contaminated energy, and renormalizes the energy to yield a corrected low-spin energy. The exchange coupling parameter in this model is given by