# Transitional state

**Transition state** is a term from chemistry . In the course of a chemical reaction , a stage of higher energy is always passed through. For example, bond angles must first be twisted and bond lengths shortened or lengthened, which is energy-consuming . The state of maximum energy is called the transition state. Once this energy barrier has been overcome, the molecule continues to react spontaneously to form the product . Transitional states are therefore of extremely short duration and cannot be isolated. The energy required for the reaction to reach the transition state is called the activation energy . According to the theory of the transition state , the transition state determines the rate constant of a reaction.

From a theoretical and chemical point of view, a reaction of a "migration" on the "mountains" corresponds to the potential energy of the nuclei (which can be calculated by various methods such as Hartree-Fock, MP2, Coupled-Cluster or DFT via an approximate solution of the Schrödinger equation ), Each arrangement of the atoms in the molecule is assigned a certain energy, the so-called potential energy hypersurface therefore depends on 3n Cartesian coordinates for n atoms in the molecule. At the transition state this energy is maximum on the reaction path, but seen globally it is a maximum in only one dimension (which corresponds to the reaction path), in all other dimensions the transition state still has a minimum (it can therefore be used as a “pass” in the above Perceive mountains). In mathematical terms, the transition state is a first order saddle point on the potential energy hypersurface, i.e. the gradient vector disappears and the Hessian matrix has a negative entry (this corresponds to an imaginary oscillation frequency in harmonic approximation).