In this section we will define the intruder state problem from a phenomenological perspective, review the theory behind the level shift techniques and introduce the σ p regularization formalism. The discussion is very general and applies, in principle, to any approach based on second-order Rayleigh-Schrödinger perturbation theory (RSPT2). However, in the following, these techniques are presented in the context of CASPT2, as this method is the focus of the present work.