1. See, e.g., Wheland GW (1955) Resonance in organic chemistry. John Wiley and Sons, New York
2. See, e.g., Mattis DC (1965) The theory of magnetism. Harper and Row, New York
3. Smart JS (1966) Effective field theories of magnetism. WB Co., Philadelphia
4. Klein DJ, Garciá-Bach MA (1979) Phys Rev B19:877
5. Klein DJ (1979) Int J Quantum Chem 13S:293