has zero off-diagonal elements in the reference space, which results in an extremely smooth PES as shown in Figure (despite the slight non-invariance introduced by the SS-SR internal contraction scheme). As ζ increases, the density matrix and the Fock operator progressively gain a state-dependent character. Consequently, the H (0) off-diagonal coupling between the rotated states may become different from zero (see Eq. ( )), potentially making XDW and RMS less smooth than XMS. In this particular case, the PES described using XDW (Figure ) is extremely smooth. On the other hand, at the ζ → ∞ limit, RMS has a few light irregularities in Figure D. The distortion in this case is up to 4 kcal/mol, but RMS remains considerably smoother than MS despite the similar state-specific Fock operator.