ABSTRACT
Rayleigh-Schrödinger (RS) resolvent based quasi-degenerate perturbation theories (QDPT) are often plagued with intruder state problems due to the near degeneracy of the model and the virtual state functions. Considerable efforts have been made over the years to formulate intruder free size extensive multireference perturbation theories for both the ground and excited states. This problem can be partly mitigated by using the Brillouin-Wigner (BW) resolvent, but then the energies will not be size extensive, even for complete active space.
