to CAS(10e,10o)). Then, the accuracy of the implemented analytic gradients is discussed using a comparison between numerical and "analytic" gradients. We first show that the two additional constraint conditions, discussed in Sections II C 3 and II C 4, are necessary for fully analytic derivatives using hydrogen fluoride with a separation of 1.60 Å at the XMS-CASPT2(2e,2o)/cc-pVDZ level of theory. The active space contained one σ and σ * orbitals, and the first two states (SA2) were averaged in the reference CASSCF calculation. A similar comparison was made using planar butadiene at the XMS-and RMS-CASPT2 or RASPT2 levels as in Ref. 36.