I have also been studying the possibilities of developing a Model Hamiltonian (MH) technique in the defect electronic structure problem. The original aim was to find a simplified second-quantisation electron-phonon Hamiltonian for a crystal with defect without using artificial approximations of the cluster model. It was found that the Landau-Pekar Ansatz for the electron-phonon wave function together with the electronic polaron theory and linear electron-phonon coupling serve as a good basis for the theory. In [B35, B46] we have studied the self-trapped hole (Vk-centre) in the KCl crystal by means of the simplified MH method and have proven the crucial importance of the short-range terms in the MH (contributions arising due to the chemical bond in the Cl-2-molecule).