I needed to have a UDF for "diffusivity". Fortunately, it's given already in the UDF Manual(4.3.3). I've copied it as such directly in a notepad file and saved it with an extension .c and saved it in the same directory where my .cas and .dbs files are being saved. However, interpretation displays a very long (and probably fundamental) error. My problem is 3D. And, oe more thing - Do we need to choose a scalar before defining a UDF ? Kindly help ... I am new user of UDFs.

Yes, you might need to hook the UDF's. But this is only after the compilation/interpreting has been successfully completed.

Thanx Pete. But Please answer specifically for each question - Do I need to create a U D Scalar ?

And, do I need to make my case Unsteady for this particular macro "Diffusivity" described in UDF manual 4.3.3 ?

You don't need to set any UDS. And the error of UDF come from bad .c code. Check your .c code if any worng statement?

And one more thing that the examples in UDF Manual may not work. Coz it's not the complete code, It's just a simple example to introduce each "Marco". So you need to write your own code.

Peace,