# Transport coefficient

A transport coefficient ${\displaystyle \gamma }$ measures how rapidly a perturbed system returns to equilibrium.

The transport coefficients occur in transport phenomenon with transport laws ${\displaystyle {\mathbf {J} {_{k}}}\,=\,\gamma _{k}\,\mathbf {X} {_{k}}}$ where:

${\displaystyle {\mathbf {J} {_{k}}}}$ is a flux of the property ${\displaystyle k}$
the transport coefficient ${\displaystyle \gamma _{k}}$ of this property ${\displaystyle k}$
${\displaystyle {\mathbf {X} {_{k}}}}$, the gradient force which acts on the property ${\displaystyle k}$.

Transport coefficients can be expressed via a Green–Kubo relation:

${\displaystyle \gamma =\int _{0}^{\infty }\langle {\dot {A}}(t){\dot {A}}(0)\rangle \,dt,}$

where ${\displaystyle A}$ is an observable occurring in a perturbed Hamiltonian, ${\displaystyle \langle \cdot \rangle }$ is an ensemble average and the dot above the A denotes the time derivative.[1] For times ${\displaystyle t}$ that are greater than the correlation time of the fluctuations of the observable the transport coefficient obeys a generalized Einstein relation:

${\displaystyle 2t\gamma =\langle |A(t)-A(0)|^{2}\rangle .}$

In general a transport coefficient is a tensor.