From a theoretical perspective, we have seen in the previous section that the imaginary shift and σ p regularization depend on the spectrum of the zeroth-order Hamiltonian. For a diagonal zeroth-order Hamiltonian, the use of any of these techniques strictly guarantees that intruder states are progressively removed by increasing the value of the input parameter. This is not the case for CASPT2, where the off-diagonal couplings in Ĥ(0) could accidentally introduce intruder states. To provide an unbiased comparison between the shifts and the regularizers, we perform the NOISP and ISP analyses using the CASPT2D approach first.