A transport coefficient measures how rapidly a perturbed system returns to equilibrium.
The transport coefficients occur in transport phenomenon with transport laws where:
- is a flux of the property
- the transport coefficient of this property
- , the gradient force which acts on the property .
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. 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.