full multicomp DIFF?
 

I am attempting to model diffusion of a species at low flows (laminar, viscous). Using v.6.2.16, I haven't been able to select the Full Multicomponent Diffusion when in the Species Model dialog box. That selection is "greyed out", but it allows me to click on the other diffusion boxes such as Diff Energy Source.

How can I enable the Full Multicomponent Diffusion???