Thermal diffusion is the separation of two species of differing size in the presence of a temperature gradient. Because there can be strong temperature gradients in a reactor, thermal diffusion can significantly influence deposition rates[86] , [91], [92] as well as density profiles, as observed by in situ measurements [93], [94], [95], [96]. The effect of thermal diffusion is included in the diffusion velocity as the second term on the right side of Equation 12–5 or Equation 12–6 . Note that a full multicomponent model is used for the computation of thermal diffusion coefficients, regardless of whether the user has specified the mixture-averaged or the multicomponent option for the calculation of the diffusion velocity. See Gas-phase Species Transport Properties for more details about this formulation.