Retinal photoisomerization is at the heart of the vision process of rhodopsins, involving an ultrafast nonadiabatic transition between S 1 and S 0 via a conical intersection. Due to the size of the retinal, to determine the impact of electron correlation treatment, various truncated models of the protonated Schiff base (PSB) have been exhaustively studied, where multireference wave function methods have often been the most successful methods to describe the complex intersection topologies and energetic barriers in the excited states. For the present study, a model of the largest retinal protonated Schiff base with six double-bonds (rPSB6) is adopted and extended multi-state second-order complete-active space perturbation theory (XMS-CASPT2) is serving as a good reference for testing the quality of adaptive-exchange XC functionals.