At last, we shall note that σ p -CASPT2 is less dependent on the expression used to obtain the energy, in contrast to the real and imaginary level shifts. For these, the difference between the energy obtained with Equation ( ) and Equation ( ) -the level-shift correction -is about 0.45 eV and 0.2 eV, respectively, for a level shift parameter ε = 0.3 E h . On the other hand, for σ 2 -CASPT2 and σ 1 -CASPT2 this is about 0.08 eV and 0.15 eV, respectively, for the same value of the input parameter ε = 0.3 E h (see Figures in the Supporting Information).