How to find nput impedance of FSS unit cell in CST MWS

Hi,

I am facing problem in finding the input impedance of a unit cell FSS in CST with FSS unit cell template. BY applying unit cell boundary conditions and floquet ports and then simulating with frequency domain solver, I get the S parameters but the input impedance is not calculated. BY simulating simultaneously with floquet ports and waveguide port, i get the input impedance but those results are different than the theoretical results.

Any help would be appreciated.
Thanks