IE3D result gets negative resistance!!!

Among the values I get in IE3D simulation, using model 2-port PI with series RL and shunt RC, I keep on getting the negative resistance of the resistor among the values during my simulation on IE3D software. Is there any problem or situation that contribute to this negative resistance as simulation result? Or is this normal and correct? Because theoretically the value of exact resistance without referring to the reactance cannot be negative, or is it because the R given in the IE3D simulation is actually equivalent impedance?

Negative resistance does not exist, although active devices (e.g. Gunn diode) can show a decreasing current with increasing voltage.

So the results you are getting are not real. Whether that's a bug or not is another matter. It might be you are using the program incorrectly, so it converges to a wrong result. However, one might expect the program to have some consistency checks, and flag up such results as bad, and hint what you might have done wrong.

I have noticed with HFSS that sometimes antenna efficiencies are above 100%, but not by very much.

Yes, that's it.

For example, if you simulate a transmission line or inductor over a lossy substrate, an extracted simple pi model will have negative series resistance at higher frequencies. That is because the simple pi model is too simplistic to describe the physical behaviour, even if the EM results are accurate.

However, you should not get negative resistance at low frequency where the pi model is appropriate.

Can you further elaborate this? I wish to understand more about how the model will result in negative resistance and is the same results obtainable by calculations. If yes, what formulas and consideration is used because as far as I know simple formula like R=rho*length/(area) does not yield negative results?

Really appreciate your help~