In this case, increasing ε always leads to increased reference weights. This is not true for CASPT2, and there are cases in which the off-diagonal couplings of the Fock operator are such that accidental degeneracies appear for ε > 0. The use of the true eigenvalues of the CASPT2 Ĥ(0) remains an open problem, and its solution is rather involved from the computational perspective because it would require the (at least partial) diagonalization of Ĥ(0) . On the positive side, the results obtained in this work for excitation energies suggest that this issue is statistically not so relevant.