Schulten-Weller radical-pair: singlet/triplet Hamiltonian H = sum A_i*I_i*S; Fubini separable yield

Schulten-Weller radical-pair mechanism for avian magneto-reception. Setup: photo-excited radical pair (e.g., the FAD-TrpH cryptochrome system) with spins S1, S2 coupled to nuclear spins I_i via isotropic hyperfine A_i. In an external…