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a treatment of chemical reactions and other processes that regards them as proceeding by a continuous change in the relative positions and potential energies of the constituent atoms and molecules. On the reaction path between the initial and final arrangements of atoms or molecules, there exists an intermediate configuration at which the potential energy has a maximum value. The configuration corresponding to this maximum is known as the activated complex, and its state is referred to as the transition state. The difference between the energies of the transition and the initial states is closely related to the experimental activation energy for the reaction; it represents the minimum energy that a reacting or flowing system must acquire for the transformation to take place. In transition-state theory, the activated complex is considered to have been formed in a state of equilibrium with the atoms or molecules in the initial state, and therefore its statistical and thermodynamic properties can be specified. The rate at which the final state is attained is determined by the number of activated complexes formed and the frequency with which they go over to the final state. These quantities may be calculated for simple systems by using statistical-mechanical principles. In this way the rate constant of a chemical or physical process may be expressed in terms of atomic and molecular dimensions, atomic masses, and interatomic or intermolecular forces. Transition-state theory can also be formulated in thermodynamic terms. (See chemical kinetics.)