A theory enabling the rate constants in chemical reactions to be calculated using statistical thermodynamics. The events assumed to be taking place can be shown in a diagram with the potential energy as the vertical axis, while the horizontal axis, called the reaction coordinate, represents the course of the reaction. As two reactants A and B approach each other, the potential energy rises to a maximum. The collection of atoms near the maximum is called the activated complex. After the atoms have rearranged in the chemical reaction, the value of the potential energy falls as the products of the reaction are formed. The point of maximum potential energy is called the transition state of the reaction, as reactants passing through this state become products. In ACT, it is assumed that the reactants are in equilibrium with the activated complex, and that this decomposes along the reaction coordinate to give the products. ACT was developed by the US chemist Henry Eyring and colleagues in the 1930s. See also Eyring equation.