Implementation of Negative Capacitance using HFSS

I am trying to implement a Non-Foster circuit implementation for an EBG structure, but with the use of lumped elements to determine the negative capacitance values. Using HFSS, can negative values be entered as capacitance values for the lumped element? And if not, is there any suggestion on what I can do to get the negative capacitance? Thanks!

It appears that the negative value can be entered, and the validation checks out. I've never tried simulating negative values, but I suspect it may work.

I tried it, it does check out and the simulation runs as well. I just wanted to be sure if it was valid and I could use the results.

To verify, I'd probably just insert a series negative capacitance in series with a transmission line, and de-embedd out the line to obtain the scattering of the capacitor on its own.

You should then be able to extract capacitance, and tell whether or not it's actually working.

two other options :

you can also check with a lumped port then connect the hfss model in the circuit part then replace it with any impedance model without resolving.
also with v18+, you can renormalize with any complex impendance as a post processing.