As the non-invariance introduced by the IPEA shift is relevant to rotations among active MOs, we can assume that the non-invariance effect may be stronger with a larger active space. To further check this hypothesis, PESs of cytosine for CI01 twist (see Section IV E) at the XMS-CASPT2(12e,9o)/cc-pVDZ level of theory with and without IPEA shift are plotted in Figure . Although the same active space as described in Section IV E was used, only two states were averaged. The PESs were quite smooth, and there were no apparent discontinuities. The size of the active space may thus be a less important factor for the smoothness of PESs.