Effective Hamiltonian theory provides a powerful framework for a rigorous construction of effective Hamiltonians from multiconfigurational many-electron wave functions computed ab initio. Pioneering works of Kato , Ôkubo , Bloch , des Cloizeaux have established the foundations of the operator effective Hamiltonian theory. Further development came from Löwdin's partitioning technique, Feshbach's formalism , and generalizations . The history and recent developments of effective Hamiltonian theories are summarized in comprehensive reviews .