The dissociation of the chromium dimer has shown that both the imaginary shift and σ 2 -CASPT2 are equally effective in removing the intruder state problem and provide a smooth potential energy curve everywhere. This is the case for the real shift too, albeit with a relatively large value of the shift parameter, which potentially has a negative impact on the overall accuracy. On the other hand, Cr 2 was the perfect example to evidence the main shortcoming of σ 1 -CASPT2 -the discontinuity at ∆ i = 0 -which limits its use for this type of applications even though it effectively removed the intruder states. We shall now turn our attention to calculations at a fixed molecular geometry, and in particular to a typical application of the CASPT2 approach: vertical transition energies. In the following, the objective is two-fold. First, we would like to evaluate the effect of σ p regularization and the level shift techniques in cases where no intruder state problems are present. In particular, we are interested in measuring the sensitivity of the results with respect to the input parameter and quantify to what extent these deviate from the unmodified CASPT2 energies. Ideally, the best technique affects these results the least, as in these situations a modification is not really needed. We call this NOISP analysis. Second, we would like to test the effectiveness of these techniques in fixing the ISP. This case is similar to the chromium dimer dissociation, where removal of singularities was necessary to obtain physical results, though, the context is different here, because we consider fixed molecular geometries. To this end, we investigate vertical excitation energies which are affected by intruder state problems.