Strong-field limit of a two-spin exchange spectrum

If the differential Zeeman energy $|w|=|{\mu }_{0}B(g_1-g_2)|$ greatly exceeds $|\Delta |$, the two mixed levels are $-\Delta /4\pm |w|/2+O({\Delta }^2/|w|)$ and approach opposite-spin product states. Four energies are generically distinct. Equal signed moments have $w=0$, so that block remains singlet/triplet rather than being unmixed by a stronger field.