Anisotropic exchange biquadratic

To model several other electronic situations (e.g., in mixed-valence compounds), the Heisenberg Hamiltonian can be augmented by specific exchange operators that accommodate, for example, double exchange, antisymmetric exchange, anisotropic exchange, or biquadratic exchange. 

Anisotropic exchange and higher-order (biquadratic S/) terms are not considered in this discussion. 

---