Intermediate Hamiltonian to avoid intruder state problems for doubly excited states

An intermediate Hamiltonian method suitable for many-body perturbation theory calculations on doubly excited states is presented. The method is designed to be able to avoid the intruder states that often prevent convergence, especially for doubly excited states. The feasibility of the procedure is demonstrated on a few doubly excited states in helium for which convergence cannot be achieved with conventional all-order many-body methods.