HFSS does not accept complex conductivities, though they can be entered. Correct representation would be to enter a negative permittivity and a conductivity for a material that exhibits a complex conductivity as a complex conductivity can easily be converted to a complex permittivity using the Lorentz-Drude Model.
given complex permittivity, er = er' + i er", then conductivity = 2*pi*Freq*e0*er" where Freq is the frequency and e0 is permittivity of free space in F/m. the conductivity entered is also just the real part of the complex conductivity.
Hope this helps!
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