gradient at faces
 

Can anyone tell me how to extract the gradient of a variable, such as the concentration gradient, at a face? I read in the manual that you should use a C_YI_RG macro, and I know how to determine the cell adjcent to a face using a F_C0 macro. When I compline my UDF I get a Argument Missmatch error. Anyone? How do you get the value of a gradient of a variable at a face?