You have two options: enable species diffusion or consider a UDS instead of species diffusion. Mass diffusivity/UDS diffusivity would be equal to D_s, and the term highlighted in red could be included by means of a source term in the species conservation equation/UDS equation.nBoth methods are essentially the same, but it is easier (and, in my opinion, more reliable) to consider species diffusion. nIn any case, I would like to mention that I have been recently dealing with a similar problem as this one (species diffusion with source terms proportional to gradients and/or laplacians) and it has been a nightmare. Source terms involving derivatives are delicate.n