σ 1 -CASPT2 is the least sensitive approach to the input parameter and effectively removes all intruder states for sufficiently large values of ε. However, due to the discontinuity of the regularized amplitudes at ∆ i = 0, its application is likely limited to calculations at a fixed molecular geometry, e.g. the determination of vertical transition energies. For calculations involving different molecular geometries, such as the dissociation of diatomic molecules or the exploration of potential energy surfaces, only the imaginary shift and σ 2 -CASPT2 ensure smooth results. This is because in these two cases, the regularized amplitudes are a continuous function of the zeroth-order energy denominator. Both approaches remove all singularities equally well, however σ 2 -CASPT2 is slightly less sensitive to the input parameter, hence our preferred choice. Nonetheless, we should note that the difference between them is probably insignificant compared to the overall accuracy offered by CASPT2 in the first place, such that in practice they both are valid options. Overall, the real level shift is the worst performer and should be avoided, due to its uniform action on all amplitudes. Only large values of the input parameter somewhat ensure no intruder states, but its results are the most sensitive to it, such that they may be considerably different from unshifted CASPT2 ones.