But both the momentum-transfer mean free path and the momentum-transfer collision frequency are difficult to calculate. Many other mean free paths can be defined. In the gas phase, λ λ is often defined as the diffusional mean free path, by assuming that a simple approximate relation is exact: D = 1 2 λ λ v , where v is the root mean square speed of the gas molecules: v = 3 k T m , where m is the mass of the diffusing species. This approximate equation becomes exact when used to define the diffusional mean free path.