However, this comes with the inherent risk that small differences in the structures used (for instance obtained with a different methodology) might significantly affect the results in the unlucky case that one or more denominators change sign. Nevertheless, one can consider that already for ε = 0.1 E h all ISPs are effectively removed by σ p -CASPT2 (and the imaginary shift), even though some states are above the (arbitrary) ∆w ref = 10 % threshold. We can see this by noting, e.g. that the energy differences are not unphysical and agree with values obtained with higher values of ε. After all, the reference weight is one possible measure to identify ISPs, and other criteria may be used as well, such as an analysis in terms of the excitation character of the perturbers with large contributions to the correlation energy.