Further, the statistical errors are generally large. Figure displays the contour plot (with other coordinates relaxed) of the PES for a typical 1,3-dipolar cycloaddition reaction N2O + C2H2 → oxadiazole as a function of the approaching distance R and the N2O bending angle θ, whose definitions are also depicted in the same figure. Starting from the initial reactants, two reactants approach each other to hit the repulsive wall, and finally arrive at the product via a narrow region around transition state, as illustrated by three representative trajectories on the contour in solid lines. However, the direct dynamics on the B3LYP/6-31G(d) surface, starting from the transition state with additional 0.6 kcal mol -1 deposited into the reaction coordinate, showed that the reaction took place mildly without hitting the repulsive wall but following the MEP. Such a typical reactive trajectory is shown in Figure by the gray (short dot) line.