Linear representation of energy-dependent Hamiltonians
  Effective (i.e., subspace-constrained) Hamiltonians become, by construction, energy-dependent while all the energy-dependent forces prove non-linear because the energy itself is merely an eigenvalue of the Hamiltonian H. One of the most natural resolutions of such a puzzle is proposed via an introduction of teh two separate linear representatives of the respective right and left action of H=H(E). Both the new energy-independent operators are non-Hermitian so that the formalism admits a natural extension to non-Hermitian initial H(E)s. 
Miloslav Znojil