Due to the presence of negative denominators around ∆ i ≈ -0.1 E h , a level shift parameter of ε = 0.1 E h is not sufficient to avoid all singularities. A value of ε = 0.2 E h significantly improves the situation, though, only at ε = 0.3 E h the obtained potential energy curve is smooth. For this last case, w ref varies only very little as a function of R, meaning that the first-order wave function provides a consistent correction at any point of the dissociation.