A curve describing the variation of the potential energy of the system of atoms that make up the reactants and products of a reaction as a function of one geometric coordinate, and corresponding to the 'energetically easiest passage' from reactants to products (i.e. along the line produced by joining the paths of steepest descent from the @T06468@ to the reactants and to the products). For an @E02035@ the relevant geometric coordinate is the @R05168@ ; for a @S05970@ it is the succession of reaction coordinates for the successive individual reaction steps. (The @R05168@ is sometimes approximated by a quasi-chemical index of reaction progress, such as 'degree of atom transfer' or @B00707@ of some specified bond.)