Gradient lines on multidimensional potential energy surfaces and chemical reaction mechanisms

We show that the path of a chemical reaction of the potential energy surface of a chemical system can only be a gradient line between two critical points. Existing rules of symmetry conservation along the reaction path are analysed to establish their consistence with published experimental data and with ab initio calculations. In some systems the possible existence of reaction paths consisting of several gradients lines (both equivalent and nonequivalent) has been demonstrated. The relationship between the reaction mechanisms and the corresponding paths on the potential energy surface is discussed. The bibliography includes 238 references.