Second, for matching values of ε, σ p -CASPT2 consistently shows smaller deviations from the reference energies compared to the imaginary level shift. Third, the RMSD and MAD increase at different rates as a function of ε for the three techniques. These results suggest that σ p -CASPT2 is less sensitive to the input parameter than CASPT2 with the imaginary shift. In particular, σ 1 shows the least deviation, highlighting its potential in this scenario.