# The main equations which define an effective Hamiltonian.
# [http://cyt-ar.com.ar/cyt-ar/images/9/95/General_Theory_of_Effective_Hamiltonians.pdf General theory of Effective Hamiltonians].
# Ab initio derivation [http://www.academia.edu/attachments/6417299/download_file Derivation of model Hamiltonians: Exchange Hamiltonian for H<sub>2</sub>].