Atransport coefficient measures how rapidly a perturbed system returns to equilibrium.
The transport coefficients occur in transport phenomenon with transport laws
where:
Transport coefficients can be expressed via a Green–Kubo relation:
where is an observable occurring in a perturbed Hamiltonian, is an ensemble average and the dot above the A denotes the time derivative.[1] For times that are greater than the correlation time of the fluctuations of the observable the transport coefficient obeys a generalized Einstein relation:
In general a transport coefficient is a tensor.
For strong gradients the transport equation typically has to be modified with higher order terms (and higher order Transport coefficients).[2]