For dynamical stereochemistry it is useful to examine the reaction path for different approach angles. We discuss the simplest representation of such input - that of the energy relief on the "reaction plane". At every distance of the attacking atom to the center of mass of the diatomic molecule and every angle of approach, it provides the minimal potential energy of the three-atom system. A plane at a given total energy E partitions the region around the molecule into cone(s) of acceptance where reaction (in classical mechanics) is possible and complementary region(s) representing the cone(s) of nonreaction. Examples illustrating different aspects of chemical behavior are provided by using potential energy functions of good quality. The systems considered include O(3P) + HCl, H + O2, Li + HF, and H + Cl2. The limitations of the representation are noted and possible refinements are indicated.