Before moving on to review the level shift techniques in CASPT2, it is important to introduce a convenient way to identify intruder states in actual calculations. Recognizing that an underlying assumption of perturbation theory is that the fluctuation potential V is a minor modification of the zeroth-order description of the system, it is expected that the first-order correction to the wave function is small compared to Ψ . A way to quantify this, is to consider the weight of Ψ (0) relative to Ψ in the wave function corrected through firstorder, Ψ = Ψ (0) + Ψ . This gives rise to the diagnostic measure called reference weight, which is given by