Single-reference methods of such as conventional Kohn-Sham (KS) DFT methods, In the singlet state, the degeneracy of the 1 B 2 and 1 A 1 states at the D 3h geometry is lifted by the lengthening of one or two C-C bonds (δ) and the torsion (φ) of one of the carbene groups along the respective C-C bond. By lengthening the two bottom C-C bonds occupied by the anti-bonding molecular orbital (MO) (1a 2 ), the 1 A 1 state is stabilized compared to 1 B 2 . On the other hand, lengthening the top single C-C bond stabilizes 1b 1 , and so does 1 B 2 , leading to the 1 P L geometry. Finally, torsion (φ) produces 1 T L , as shown in Fig. . Accordingly, the potential energy surfaces (PESs) of D 3h → 1 P L → 1 T L by MRSF, UMRSF, and U-SF-TDDFT are generated by geodesic interpolations along the three points based on optimized geometries by MRSF (Fig. ).