The smoothness of the PES described by MRPTs is important, in particular, for locating MECIs, but also in general for molecular structure optimizations. Here, MS-, XMS-, XDW-, and RMS-CASPT2 calculations were performed using a three-state averaged CASSCF(6e,4o) reference for the MECI of pyramidalized ethylene (structure (b) in Ref. 64). First, Figure shows the PES of S 0 and S 1 around the MECI located using the individual CASPT2 methods.