In fact, for a given partitioning of the Hamiltonian, a renormalization of the first-order amplitudes can be interpreted as a way to introduce correlation effects from higher-order terms. This has the net impact of decreasing the amount of electron correlation captured by second-order perturbation theory, which is typically overestimated with respect to the infinite-order limit. Within this perspective, one could determine an optimal value for ε by comparing to experiment or high-level computational reference data, essentially defining a new separate methodology that is free of intruder states by design and potentially with a better accuracy. This is for instance the rationale behind regularized MP2 as proposed by Head-Gordon and coworkers, and what has been done with the intruder state avoidance technique in MRMP2. Nevertheless, the simple functional form of the shifts and regularizers cannot compensate the limitations inherent to a second-order perturbation theory framework. For instance, whereas a value of ε = 1.15 E h for σ 2 -CASPT2 is such that the PEC of the chromium dimer matches the experimental one around the equilibrium, this comes at the cost of a far worse agreement at longer inter-nuclear distances (see Figure in the Supporting Information). This is because the effect of the regularizer is different at different correlation regimes, and there is no guarantee that a given value of ε is adequate everywhere. In light of this, our take on σ p -CASPT2 is for an approach that can be used routinely with a (small) fixed value of ε > 0 which fixes the intruder states, but that provides results as similar as possible to an unmodified version of CASPT2.