As discussed above, the IPEA shift breaks the non-invariant character of CASPT2, and thus the transformation from non-orthogonal to orthogonal ICB or from orthogonal ICB to ICC is not unique. Although both transformation steps are relevant, we can separately discuss two modifications for each transformation step. Before discussing the solution, we need to define the Lagrangian used in this study. In addition, the approach analytic derivatives are computed using the Lagrangian method 62 , which is also briefly reviewed.