Potential energy surfaces are commonly shown as three-dimensional graphs, but they can also be represented by two-dimensional graphs, in which the advancement of the reaction is plotted by the use of isoenergetic lines. The collinear system H + H 2 is a simple reaction that allows a two-dimension PES to be plotted in an easy and understandable way. In this reaction, a hydrogen atom (H) reacts with a dihydrogen molecule (H 2) by forming a new bond with one atom from the molecule, which in turn breaks the bond of the original molecule. This is symbolized as H a + H b –H c → H a –H b + H c. The progression of the reaction from reactants (H+H₂) to products (H-H-H), as well as the energy of the species that take part in the reaction, are well defined in the corresponding potential energy surface. Energy profiles describe potential energy as a function of geometrical variables (PES in any dimension are independent of time and temperature).