For the other three techniques, the results are almost identical to those in Table , and thus the same discussion holds. This is the case for the ISP analysis as well, with CASPT2 results Overall, the NOISP and ISP analyses suggest that the imaginary level shift and σ p regularization have a comparable performance. On the one hand, the former is more sensitive to the level shift parameter, but appears to be slightly more robust in solving the ISP. On the other hand, σ p -CASPT2 provides the smallest deviations from the reference energies in the NOISP analysis, in particular with p = 1. According to this latter result only, it appears that σ 1 -CASPT2 would be the best choice for calculations at a fixed molecular geometry.