Another famous shift used in CASPT2 that requires some attention is the IPEA shift. While it is technically a shift, its nature is fundamentally different from the techniques discussed in this contribution. The IPEA shift acts on the generalized Fock operator expressed in the molecular orbital basis and was introduced to fix a systematic energy underestimation of open-shell electronic configurations with respect to closed-shell ones. While this has in general a positive effect on the ISP, that is, the number of ISPs is generally smaller with the IPEA shift than without, it is not its intended use. The techniques discussed in this work act on the zeroth-order Hamiltonian expressed in the internally contracted basis spanning the FOIS. This directly targets the energies appearing in the denominators of Equation ( ), whereas the IPEA shift has a more complicated indirect effect on them due to the internal contraction formalism. In other words, the IPEA shift modifies the energies ϵ i of Equation (5) in non-trivial ways that depend on the generalized Fock eigenvalues. Given this difference, a detailed account of the IPEA shift is omitted from the current manuscript.