Editing Quantum chemistry (section)

== Chemical dynamics ==
A further step can consist of solving the [[Schrödinger equation]] with the total [[molecular Hamiltonian]] in order to study the motion of molecules. Direct solution of the Schrödinger equation is called ''[[quantum dynamics]]'', whereas its solution within the [[Semiclassical physics|semiclassical]] approximation is called ''semiclassical dynamics.'' Purely [[Classical mechanics|classical]] simulations of molecular motion are referred to as ''[[molecular dynamics]] (MD)''. Another approach to dynamics is a hybrid framework known as ''[[mixed quantum-classical dynamics]];'' yet another hybrid framework uses the [[Path integral formulation|Feynman path integral]] formulation to add quantum corrections to molecular dynamics, which is called [[path integral molecular dynamics]]. Statistical approaches, using for example classical and quantum [[Monte Carlo method]]s,  are also possible and are particularly useful for describing equilibrium distributions of states.

=== Adiabatic chemical dynamics ===
{{main|Born–Oppenheimer approximation}}
In adiabatic dynamics, interatomic interactions are represented by single [[Scalar (physics)|scalar]] [[potential]]s called [[potential energy surface]]s. This is the [[Born–Oppenheimer approximation]] introduced by [[Max Born|Born]] and [[Robert Oppenheimer|Oppenheimer]] in 1927. Pioneering applications of this in chemistry were performed by Rice and Ramsperger in 1927 and Kassel in 1928, and generalized into the [[RRKM]] theory in 1952 by [[Rudolph A. Marcus|Marcus]] who took the [[transition state]] theory developed by [[Henry Eyring (chemist)|Eyring]] in 1935 into account. These methods enable simple estimates of unimolecular [[reaction rates]] from a few characteristics of the potential surface.

=== Non-adiabatic chemical dynamics ===
{{main|Vibronic coupling}}

Non-adiabatic dynamics consists of taking the interaction between several coupled potential energy surfaces (corresponding to different electronic [[quantum state]]s of the molecule). The coupling terms are called vibronic couplings. The pioneering work in this field was done by [[Ernst Stueckelberg|Stueckelberg]], [[Lev Davidovich Landau|Landau]], and [[Clarence Zener|Zener]] in the 1930s, in their work on what is now known as the [[Landau–Zener transition]]. Their formula allows the transition probability between two [[adiabatic]] potential curves in the neighborhood of an [[avoided crossing]] to be calculated. [[Spin forbidden reactions|Spin-forbidden reactions]] are one type of non-adiabatic reactions where at least one change in [[Spin states (d electrons)|spin state]] occurs when progressing from [[Reagent|reactant]] to [[Product (chemistry)|product]].