The presence of this CI has important consequences for the ground state PES of electrocyclic ring-opening reactions. Figure shows an example of the projection of the groundstate (S0) PES of 1,3-cyclohexadiene (CHD) onto two reaction coordinates, f1 and f2, the two dihedral angles that distinguish the conrotatory (photochemically allowed) and disrotatory (thermally allowed) pathways. The PES is calculated at the α-CASSCF(6,4)/6-31G* level of theory , by constraining a range of the f1 and f2 values while relaxing the rest of the molecular geometry. The reacting C-C bond distance, RCC, was also constrained according to its values along the conrotatory minimum energy path (MEP) and the disrotatory MEP.