= Positronium Zeeman spectrum
{title2=$E_\pm=-A/4\pm\tfrac12\sqrt{A^2+16\mu_B^2B^2}$}
For the dimensionless-spin <Hamiltonian> $A\mathbf S_e\cdot\mathbf S_p+2\mu_BB(S_{ez}-S_{pz})$, the two aligned-spin states remain at $A/4$. The two states of total <spin> projection zero mix and have the displayed energies. The parameter $4\mu_BB/A$ compares off-diagonal Zeeman mixing in the singlet-triplet basis with half the zero-field splitting. A crossover is gradual, not a level-crossing <phase transition>.
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