To this end, we start by showing in Figure the CASPT2 PEC obtained with and without the real level shift, for an increasing value of ε. In the unmodified CASPT2 results, there are several places along the curve where the energy is clearly unphysical, and these are all associated to low reference weights; a typical signature of intruder states. This can be clearly seen from Figure , where we plot w ref as a function of the inter-nuclear distance R.