Figure 1: (a) A schematic one-dimensional curve of two coupled adiabatic PESs, (b) an example of for the PESs in (a), and (c) comparisons between stationary points on (with kJ/mol) and those on the S_{1}-PES of H_{2}CO (see Section 2.4 for the ab initio calculation level). Each structure shown in (c) corresponds to S_{1}-MIN, S_{1}-TS, and S_{0}/S_{1}-MSX (conical intersection), respectively, and stationary structures on are overlapped in black behind them. At S_{1}-MIN and S_{1}-TS, the stationary structures on are identical to them within the convergence criteria (see text) of geometry optimization and cannot be seen. -MIN can barely be seen behind S_{0}/S_{1}-MSX.