The MS-CASPT2 LIIC-PES is given in section S5 of SI. To understand this phenomenon in detail, the MEP for Ds along C 12 -C 3 -C 2 -C 8 dihedral angle given in Fig. The 2D surfaces around MECP of Ds molecule is shown in Fig. ). The surface is tilted towards -⃗ h-vector and it is symmetrical about +⃗ g-vector. The tilt towards -⃗ h-vector corresponds the tilt towards S 0 . Now the + ⃗ h-vector corresponds to a slow in-plane motion where +⃗ g-vector corresponds to out-of-plane motion. As the surface is tilted towards -⃗ hvector which is again tilted towards S 0 , therefore after reaching MECP, the molecule will have a higher probability of photo-protection i.e. it will return back to ground state.