How to do theoretical calculation of S parameters
The long answer is set up the problem like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Now calculate the forward and reverse-travelling waves at each of the ports. This gives you \$S_{11}\$ and \$S_{21}\$. 

Then hook up the generator on the other side of the device and calculate the forward and reverse travelling waves again to get \$S_{12}\$ and \$S_{22}\$.

But since you are already using ADS, there is an easier way: Hook up your device model between S parameter ports and simulate it. This will also make it easier to make ADS draw a graph comparing the simple model with the more complex model.

If your "theoretical" model is a simple combination of R, L, and C elements, you could even use the ADS optimizer to tune the theoretical model to match the phenomenological model (or vice versa), or to match some measurement data.