is possible in CASPT2 that energy denominators take on negative values and accidentally fall near the singularity, that is ∆ i ≈ -ε. This is particularly problematic when exploring several conformations, as it can be hard to find a suitable range for ε that avoids all singularities, or when computing excited states and investigating transition metals, where a large a number of small denominators make the results very sensitive to the value of the parameter. In an attempt at solving the shortcomings of the real level shift, Forsberg and