Many-body perturbation theory
In our equations for \( \vert \Psi_0\rangle \) and \( \Delta E \) in terms of the unperturbed
solutions \( \vert \Phi_i\rangle \) we have still an undetermined parameter \( \omega \)
and a dependecy on the exact energy \( E \). Not much has been gained thus from a practical computational point of view.