As second showcase example, we studied the methylenimmonium cation (CH 2 NH + 2 ), which is the smallest member of the protonated Schiff bases that has been used as model systems to study the fundamental processes involved in vision. To this end, we relied on the literature-known database of CH 2 NH + 2 comprising 4 k data points at the multi-reference configuration interaction singles and doubles (MR-CISD) level of theory with an active space of four active electrons in three active orbitals, namely MR-CISD (4,3). This data has served as a good test bed for several developments as it is characterized by ultrafast internal conversion after excitation to the second excited singlet state.