Fig. 3: Energy landscape (in Hartree) of ethylene plotted against two circuit parameters θ0 and θ1 (in radian).

The energies of ethylene were computed at the FC structure using the CASCI/STO-3G method, whose active space included two electrons in two orbitals. One of the minimum energy points (S0), the first-order saddle point (S1), and the second-order saddle point (S2) are shown by the white circle, black x, and black triangle, respectively. The white solid line represents the region where the wavefunction is orthogonal to S0.